This change adds a better implementation of IndexByte in asm that uses the
vector registers/instructions on ppc64x.

benchmark                            old ns/op     new ns/op     delta
BenchmarkIndexByte/10-8              9.70          9.37          -3.40%
BenchmarkIndexByte/32-8              10.9          10.9          +0.00%
BenchmarkIndexByte/4K-8              254           92.8          -63.46%
BenchmarkIndexByte/4M-8              249246        118435        -52.48%
BenchmarkIndexByte/64M-8             10737987      7383096       -31.24%

benchmark                            old MB/s     new MB/s     speedup
BenchmarkIndexByte/10-8              1030.63      1067.24      1.04x
BenchmarkIndexByte/32-8              2922.69      2928.53      1.00x
BenchmarkIndexByte/4K-8              16065.95     44156.45     2.75x
BenchmarkIndexByte/4M-8              16827.96     35414.21     2.10x
BenchmarkIndexByte/64M-8             6249.67      9089.53      1.45x

Change-Id: I81dbdd620f7bb4e395ce4d1f2a14e8e91e39f9a1
Reviewed-on: https://go-review.googlesource.com/71710
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>