eqstring is only called for strings with equal lengths.
Instead of pushing a pointer and length for each argument string
on the stack we can omit pushing one of the lengths on the stack.

Changing eqstrings signature to eqstring(*uint8, *uint8, int) bool
to implement the above optimization would make it very similar to the
existing memequal(*any, *any, uintptr) bool function.

Since string lengths are positive we can avoid code redundancy and
use memequal instead of using eqstring with an optimized signature.

go command binary size reduced by 4128 bytes on amd64.

name                          old time/op    new time/op    delta
CompareStringEqual              6.03ns ± 1%    5.71ns ± 1%   -5.23%  (p=0.000 n=19+18)
CompareStringIdentical          2.88ns ± 1%    3.22ns ± 7%  +11.86%  (p=0.000 n=20+20)
CompareStringSameLength         4.31ns ± 1%    4.01ns ± 1%   -7.17%  (p=0.000 n=19+19)
CompareStringDifferentLength    0.29ns ± 2%    0.29ns ± 2%     ~     (p=1.000 n=20+20)
CompareStringBigUnaligned       64.3µs ± 2%    64.1µs ± 3%     ~     (p=0.164 n=20+19)
CompareStringBig                61.9µs ± 1%    61.6µs ± 2%   -0.46%  (p=0.033 n=20+19)

Change-Id: Ice15f3b937c981f0d3bc8479a9ea0d10658ac8df
Reviewed-on: https://go-review.googlesource.com/53650
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>