[dev.simd] simd, cmd/compile: sample peephole optimization for .Masked()

[dev.simd] simd, cmd/compile: sample peephole optimization for .Masked()

This is not the end of such peephole optimizations, there
would need to be many of these for many simd operations.

Change-Id: I4511f6fac502bc7259c1c4414c96f56eb400c202
Reviewed-on: https://go-review.googlesource.com/c/go/+/697157
TryBot-Bypass: David Chase <drchase@google.com>
Commit-Queue: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>