[dev.simd] simd, cmd/compile: added .Masked() peephole opt for many operations.

[dev.simd] simd, cmd/compile: added .Masked() peephole opt for many operations.

This should get many of the low-hanging and important fruit.
Others can follow later.
It needs more testing.

Change-Id: Ic186b075987e85c87197ef9e1ca0b4f33ff96697
Reviewed-on: https://go-review.googlesource.com/c/go/+/697515
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Commit-Queue: David Chase <drchase@google.com>
TryBot-Bypass: David Chase <drchase@google.com>