[dev.simd] cmd/compile: add register-to-mask moves, other simd glue

[dev.simd] cmd/compile: add register-to-mask moves, other simd glue

This includes code generated by simdgen CL 689955,
here because of git-facilitated pilot error
(the generated file should have been in the next CL
but that is related to this one, so, oh well).

Change-Id: Ibfea3f1cd93ca9cd12970edf15a013471677a6ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/689936
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>