CmmSink: Use a IntSet instead of a list
CmmSink: Use a IntSet instead of a list

CmmProcs which have *lots* of local variables take a considerable
amount of time in CmmSink. This was noticed by @tdammers in #7258
while compiling files with large records (~200-400 fields).

Before:

```
        Sun Oct 29 19:58 2017 Time and Allocation Profiling Report (Final)

           ghc-stage2 +RTS -p -RTS
-B/Users/alexbiehl/git/ghc/inplace/lib /Users/alexbiehl/Downloads/W2.hs
-fforce-recomp -O2

        total time  =       26.00 secs   (25996 ticks @ 1000 us, 1 processor)
        total alloc = 14,921,627,912 bytes  (excludes profiling overheads)

COST CENTRE     MODULE      SRC %time %alloc

sink            CmmPipeline
compiler/cmm/CmmPipeline.hs:(104,13)-(105,59)        55.7   15.9
SimplTopBinds   SimplCore   compiler/simplCore/SimplCore.hs:761:39-74 19.5   30.6
FloatOutwards   SimplCore   compiler/simplCore/SimplCore.hs:471:40-66 4.2    9.0
RegAlloc-linear AsmCodeGen compiler/nativeGen/AsmCodeGen.hs:(658,27)-(660,55)    4.0   11.1
pprNativeCode   AsmCodeGen compiler/nativeGen/AsmCodeGen.hs:(529,37)-(530,65)    2.8    6.3
NewStranal      SimplCore   compiler/simplCore/SimplCore.hs:480:40-63 1.6    3.7
OccAnal         SimplCore compiler/simplCore/SimplCore.hs:(739,22)-(740,67)     1.5    3.5
StgCmm          HscMain compiler/main/HscMain.hs:(1426,13)-(1427,62)          1.2    2.4
regLiveness     AsmCodeGen compiler/nativeGen/AsmCodeGen.hs:(591,17)-(593,52)    1.2    1.9
genMachCode     AsmCodeGen compiler/nativeGen/AsmCodeGen.hs:(580,17)-(582,62)    0.9    1.8
NativeCodeGen   CodeOutput  compiler/main/CodeOutput.hs:171:18-78 0.9    2.1
CoreTidy        HscMain     compiler/main/HscMain.hs:1253:27-67 0.8    1.9
```

After:

```
        Sun Oct 29 19:18 2017 Time and Allocation Profiling Report (Final)

           ghc-stage2 +RTS -p -RTS
-B/Users/alexbiehl/git/ghc/inplace/lib /Users/alexbiehl/Downloads/W2.hs
-fforce-recomp -O2

        total time  =       13.31 secs   (13307 ticks @ 1000 us, 1 processor)
        total alloc = 15,772,184,488 bytes  (excludes profiling overheads)

COST CENTRE     MODULE         SRC %time %alloc

SimplTopBinds   SimplCore
compiler/simplCore/SimplCore.hs:761:39-74            38.3   29.0
sink            CmmPipeline compiler/cmm/CmmPipeline.hs:(104,13)-(105,59)        13.2   20.3
RegAlloc-linear AsmCodeGen compiler/nativeGen/AsmCodeGen.hs:(658,27)-(660,55)    8.3   10.5
FloatOutwards   SimplCore compiler/simplCore/SimplCore.hs:471:40-66             8.1    8.5
pprNativeCode   AsmCodeGen compiler/nativeGen/AsmCodeGen.hs:(529,37)-(530,65)    5.4    5.9
NewStranal      SimplCore compiler/simplCore/SimplCore.hs:480:40-63             3.1    3.5
OccAnal         SimplCore compiler/simplCore/SimplCore.hs:(739,22)-(740,67)     2.9    3.3
StgCmm          HscMain compiler/main/HscMain.hs:(1426,13)-(1427,62)          2.3    2.3
regLiveness     AsmCodeGen compiler/nativeGen/AsmCodeGen.hs:(591,17)-(593,52)    2.1    1.8
NativeCodeGen   CodeOutput     compiler/main/CodeOutput.hs:171:18-78 1.7    2.0
genMachCode     AsmCodeGen compiler/nativeGen/AsmCodeGen.hs:(580,17)-(582,62)    1.6    1.7
CoreTidy        HscMain        compiler/main/HscMain.hs:1253:27-67 1.4    1.8
foldNodesBwdOO  Hoopl.Dataflow compiler/cmm/Hoopl/Dataflow.hs:(397,1)-(403,17)       1.1    0.8
```

Reviewers: austin, bgamari, simonmar

Reviewed By: bgamari

Subscribers: duog, rwbarton, thomie, tdammers

GHC Trac Issues: #7258

Differential Revision: https://phabricator.haskell.org/D4145

shlevy/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc