runtime: fix regression in BenchmarkCompareBytes* for ppc64x

runtime: fix regression in BenchmarkCompareBytes* for ppc64x

Between go1.7 and go1.8, a performance regression was introduced in some of the
BenchmarkCompareBytes benchmarks.

Go1.7 vs Go1.8:
BenchmarkCompareBytesToNil-8               7.44          8.44          +13.44%
BenchmarkCompareBytesIdentical-8           6.96          11.5          +65.23%
BenchmarkCompareBytesBigIdentical-8        6.65          47112         +708351.13%

This change fixes the problem by optimizing the case where the byte slices being
compared are equal:

Go1.9 vs current:
BenchmarkCompareBytesToNil-8               7.35          7.00          -4.76%
BenchmarkCompareBytesIdentical-8           11.4          6.81          -40.26%
BenchmarkCompareBytesBigIdentical-8        48396         9.26          -99.98%

runtime.cmpstring can benefit from the same approach and is also changed.

Change-Id: I3cb25f59d8b940a83a2cf687eea764cfeff90688
Reviewed-on: https://go-review.googlesource.com/59650
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>