[yaml::BinaryRef] Slight perf tuning (for llvm-exegesis analysis mode)
[yaml::BinaryRef] Slight perf tuning (for llvm-exegesis analysis mode)

Summary:
llvm-exegesis uses this functionality to read it's benchmark dumps.
This reading of `.yaml`s takes ~60% of runtime for 14656 benchmark points (i.e. one sweep over all x86 instructions),
but only 30% of time for 3x as much benchmark points.

In particular, this `BinaryRef` appears to be an obvious pain point.
Without patch:
```
$ perf stat -r 25 ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=1.0 -benchmarks-file=/tmp/benchmarks-inverse_throughput-onefull.yaml -analysis-clusters-output-file="" -analysis-inconsistencies-output-file=/tmp/clusters-orig.html
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 14656 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-orig.html'
...
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 14656 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-orig.html'

 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=1.0 -benchmarks-file=/tmp/benchmarks-inverse_throughput-onefull.yaml -analysis-clusters-output-file= -analysis-inconsistencies-output-file=/tmp/clusters-orig.html' (25 runs):

            972.86 msec task-clock                #    0.994 CPUs utilized            ( +-  0.25% )
                30      context-switches          #   30.774 M/sec                    ( +- 21.74% )
                 0      cpu-migrations            #    0.370 M/sec                    ( +- 67.81% )
             11873      page-faults               # 12211.512 M/sec                   ( +-  0.00% )
        3898373408      cycles                    # 4009682.186 GHz                   ( +-  0.25% )  (83.12%)
         360399748      stalled-cycles-frontend   #    9.24% frontend cycles idle     ( +-  0.54% )  (83.24%)
        1099450483      stalled-cycles-backend    #   28.20% backend cycles idle      ( +-  0.59% )  (33.63%)
        4910528820      instructions              #    1.26  insn per cycle
                                                  #    0.22  stalled cycles per insn  ( +-  0.13% )  (50.21%)
        1111976775      branches                  # 1143726625.854 M/sec              ( +-  0.10% )  (66.77%)
          23248474      branch-misses             #    2.09% of all branches          ( +-  0.19% )  (83.29%)

           0.97850 +- 0.00647 seconds time elapsed  ( +-  0.66% )
```
With the patch:
```
$ perf stat -r 25 ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=1.0 -benchmarks-file=/tmp/benchmarks-inverse_throughput-onefull.yaml -analysis-clusters-output-file="" -analysis-inconsistencies-output-file=/tmp/clusters-new.html
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 14656 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-new.html'
...
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 14656 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-new.html'

 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=1.0 -benchmarks-file=/tmp/benchmarks-inverse_throughput-onefull.yaml -analysis-clusters-output-file= -analysis-inconsistencies-output-file=/tmp/clusters-new.html' (25 runs):

            905.29 msec task-clock                #    0.999 CPUs utilized            ( +-  0.11% )
                15      context-switches          #   16.533 M/sec                    ( +- 32.27% )
                 0      cpu-migrations            #    0.000 K/sec
             11873      page-faults               # 13121.789 M/sec                   ( +-  0.00% )
        3627759720      cycles                    # 4009283.100 GHz                   ( +-  0.11% )  (83.19%)
         370401480      stalled-cycles-frontend   #   10.21% frontend cycles idle     ( +-  0.22% )  (83.19%)
        1007114438      stalled-cycles-backend    #   27.76% backend cycles idle      ( +-  0.34% )  (33.62%)
        4414014304      instructions              #    1.22  insn per cycle
                                                  #    0.23  stalled cycles per insn  ( +-  0.08% )  (50.36%)
        1003751700      branches                  # 1109314021.971 M/sec              ( +-  0.07% )  (66.97%)
          24611010      branch-misses             #    2.45% of all branches          ( +-  0.10% )  (83.41%)

           0.90593 +- 0.00105 seconds time elapsed  ( +-  0.12% )
```
So this decreases the overall run time of llvm-exegesis analysis mode (on one sweep) by roughly -7%.

To be noted, `BinaryRef::writeAsBinary()` change is the reason for the perf changes,
usage of `llvm::isHexDigit()` instead of `isxdigit()` does not appear to have any perf impact,
i have only changed it "for symmetry".

`writeAsBinary()` change is correct, it produces identical de-hex-ified buffer, and the final output is thus identical:
```
$ sha512sum /tmp/clusters-*
db4bbd904fe8840853b589b032c5041bc060b91bcd9c27b914b56581fbc473550eea74b852238c79963b5adf2419f379e9f5db76784048b48e3937f9f3e732bf  /tmp/clusters-new.html
db4bbd904fe8840853b589b032c5041bc060b91bcd9c27b914b56581fbc473550eea74b852238c79963b5adf2419f379e9f5db76784048b48e3937f9f3e732bf  /tmp/clusters-orig.html
```

Reviewers: silvas, espindola, sbc100, zturner, courbet, gchatelet

Reviewed By: gchatelet

Subscribers: tschuett, RKSimon, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57699

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@353282 91177308-0d34-0410-b5e6-96231b3b80d8

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