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

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

name             old time/op  new time/op  delta
Add-8            1.11ns ± 0%  1.18ns ± 0%   +6.31%  (p=0.029 n=4+4)
Add32-8          1.02ns ± 0%  1.02ns ± 1%     ~     (p=0.333 n=4+5)
Add64-8          1.11ns ± 1%  1.17ns ± 0%   +5.79%  (p=0.008 n=5+5)
Add64multiple-8  4.35ns ± 1%  0.86ns ± 0%  -80.22%  (p=0.000 n=5+4)

The individual ops are a bit slower (but still very fast).
Using the ops in carry chains is very fast.

Update #28273

Change-Id: Id975f76df2b930abf0e412911d327b6c5b1befe5
Reviewed-on: https://go-review.googlesource.com/c/144257
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>