cmd/compile: intrinsify Sub64 on mips64

cmd/compile: intrinsify Sub64 on mips64

This CL intrinsify Sub64 on mips64.

pkg: math/bits
                  _   sec/op    _   sec/op     vs base               _
Sub-4               2.849n _ 0%   1.948n _ 0%  -31.64% (p=0.000 n=8)
Sub32-4             3.447n _ 0%   3.446n _ 0%        ~ (p=0.982 n=8)
Sub64-4             2.815n _ 0%   1.948n _ 0%  -30.78% (p=0.000 n=8)
Sub64multiple-4     6.124n _ 0%   3.340n _ 0%  -45.46% (p=0.000 n=8)

Change-Id: Ibba91a4350e4a549ae0b60d8cafc4bca05034b84
Reviewed-on: https://go-review.googlesource.com/c/go/+/498497
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Joel Sing <joel@sing.id.au>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>