[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>

src/cmd/compile/internal/amd64/ssa.go

@@ -1671,6 +1671,8 @@ func simdReg(v *ssa.Value) int16 {
 		panic("simdReg: not a simd type")
 	}
 	switch t.Size() {
+	case 8:
+		return v.Reg() // K registers
 	case 16:
 		return v.Reg()
 	case 32:

src/cmd/compile/internal/types/type.go

@@ -1633,7 +1633,7 @@ var (
 	TypeVec128    = newSIMD("vec128")
 	TypeVec256    = newSIMD("vec256")
 	TypeVec512    = newSIMD("vec512")
-	TypeMask      = newSSA("mask") // not a vector, not 100% sure what this should be.
+	TypeMask      = newSIMD("mask") // not a vector, not 100% sure what this should be.
 	TypeResultMem = newResults([]*Type{TypeMem})
 )