internal/bytealg: optimize indexbyte function for ppc64le/power9
internal/bytealg: optimize indexbyte function for ppc64le/power9

Added specific code For POWER9 that does not need prealignment prior
to load vector. Optimized vector loop to jump out as soon as there is
a match instead of accumulating matches for 4 indices and then processing
the same. For small input size 10, the caller function dominates
performance.

name                      old time/op    new time/op    delta
IndexByte/10                9.20ns ± 0%   10.40ns ± 0%  +13.08%
IndexByte/32                9.77ns ± 0%    9.20ns ± 0%   -5.84%
IndexByte/4K                 171ns ± 0%     136ns ± 0%  -20.51%
IndexByte/4M                 154µs ± 0%     126µs ± 0%  -17.92%
IndexByte/64M               2.48ms ± 0%    2.03ms ± 0%  -18.27%
IndexAnyASCII/1:32          10.2ns ± 1%     9.2ns ± 0%   -9.19%
IndexAnyASCII/1:64          11.3ns ± 0%    10.1ns ± 0%  -11.29%
IndexAnyUTF8/1:64           11.4ns ± 0%     9.8ns ± 0%  -13.73%
IndexAnyUTF8/16:64           156ns ± 1%     131ns ± 0%  -16.23%
IndexAnyUTF8/256:64         2.27µs ± 0%    1.86µs ± 0%  -18.03%
LastIndexAnyUTF8/1:64       11.8ns ± 0%    10.5ns ± 0%  -10.81%
LastIndexAnyUTF8/16:64       165ns ±11%     132ns ± 0%  -19.75%
LastIndexAnyUTF8/256:2      1.68µs ± 0%    1.44µs ± 0%  -14.33%
LastIndexAnyUTF8/256:4      1.68µs ± 0%    1.49µs ± 0%  -11.10%
LastIndexAnyUTF8/256:8      1.68µs ± 0%    1.50µs ± 0%  -11.05%
LastIndexAnyUTF8/256:64     2.30µs ± 0%    1.90µs ± 0%  -17.56%
Change-Id: I3d2550bdfdea38fece2da9960bbe62fe6cb1840c
Reviewed-on: https://go-review.googlesource.com/c/go/+/397614
Reviewed-by: Paul Murphy <2e1d8d6f81223fe22f04c5349bc2b02555a275af@ibm.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Archana Ravindar <f682d884382604a3c853f1eb5e063792ab430218@in.ibm.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go,golang/go,golang/go,nawawi/go,golang/go,golang/go,nawawi/go,golang/go