cmd/compile: intrinsify math/bits.Sub on amd64

cmd/compile: intrinsify math/bits.Sub on amd64

name             old time/op  new time/op  delta
Sub-8            1.12ns ± 1%  1.17ns ± 1%   +5.20%          (p=0.008 n=5+5)
Sub32-8          1.11ns ± 0%  1.11ns ± 0%     ~     (all samples are equal)
Sub64-8          1.12ns ± 0%  1.18ns ± 1%   +5.00%          (p=0.016 n=4+5)
Sub64multiple-8  4.10ns ± 1%  0.86ns ± 1%  -78.93%          (p=0.008 n=5+5)

Fixes #28273

Change-Id: Ibcb6f2fd32d987c3bcbae4f4cd9d335a3de98548
Reviewed-on: https://go-review.googlesource.com/c/144258
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>