[dev.simd] cmd/compile: add and fix k register supports

[dev.simd] cmd/compile: add and fix k register supports

This CL marks the "mask" ssa type as a simd type. This will make the
last return of `simdMov` reachable and the spilling of K register
correct.

This CL also makes `simdReg` able to return K registers.

Change-Id: Ia66230d3e5425d9e8bdd0081b008e098382d3827
Reviewed-on: https://go-review.googlesource.com/c/go/+/676876
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>