internal/bytealg: optimize index function for ppc64le/power9

Optimized index2to16 loop by unrolling the loop by 4.
Multiple benchmark tests show performance improvement on
POWER9. Similar improvements are seen on POWER10. Added
tests to ensure changes work fine.

name            old time/op   new time/op    delta
Index/10         18.3ns ± 0%    19.7ns ±25%     ~
Index/32         75.3ns ± 0%    69.2ns ± 0%   -8.22%
Index/4K         5.53µs ± 0%    3.69µs ± 0%  -33.20%
Index/4M         5.64ms ± 0%    3.75ms ± 0%  -33.55%
Index/64M        92.9ms ± 0%    61.6ms ± 0%  -33.69%
IndexHard2       1.41ms ± 0%    0.93ms ± 0%  -33.75%
CountHard2       1.41ms ± 0%    0.93ms ± 0%  -33.75%

Change-Id: If9331df6a141a4716724b8cb648d2b91bdf17e5f
Reviewed-on: https://go-review.googlesource.com/c/go/+/377016
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Archana Ravindar <aravind5@in.ibm.com>

src/bytes/bytes_test.go

@@ -140,6 +140,36 @@ var indexTests = []BinOpTest{
 	{"abc", "c", 2},
 	{"abc", "x", -1},
 	{"barfoobarfooyyyzzzyyyzzzyyyzzzyyyxxxzzzyyy", "x", 33},
+	{"fofofofooofoboo", "oo", 7},
+	{"fofofofofofoboo", "ob", 11},
+	{"fofofofofofoboo", "boo", 12},
+	{"fofofofofofoboo", "oboo", 11},
+	{"fofofofofoooboo", "fooo", 8},
+	{"fofofofofofoboo", "foboo", 10},
+	{"fofofofofofoboo", "fofob", 8},
+	{"fofofofofofofoffofoobarfoo", "foffof", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffof", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofo", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofo", 13},
+	{"fofofofofoofofoffofoobarfoo", "foffofoo", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofoo", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofoob", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofoob", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofooba", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofooba", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofoobar", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofoobar", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofoobarf", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofoobarf", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofoobarfo", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofoobarfo", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofoobarfoo", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofoobarfoo", 12},
+	{"fofofofofoofofoffofoobarfoo", "ofoffofoobarfoo", 12},
+	{"fofofofofofofoffofoobarfoo", "ofoffofoobarfoo", 11},
+	{"fofofofofoofofoffofoobarfoo", "fofoffofoobarfoo", 11},
+	{"fofofofofofofoffofoobarfoo", "fofoffofoobarfoo", 10},
+	{"fofofofofoofofoffofoobarfoo", "foobars", -1},
 	{"foofyfoobarfoobar", "y", 4},
 	{"oooooooooooooooooooooo", "r", -1},
 	{"oxoxoxoxoxoxoxoxoxoxoxoy", "oy", 22},