[dev.simd] cmd/compile: mark SIMD types non-fat

This CL fixes the merge locals error.
The culprit is that liveness analysis wrongly mark SIMD structs fat,
hence making `StoreReg` of SIMD vectors not a varkill effect, making the
liveness range of SIMD vectors not closed correctly, further making
mergelocals merged 2 concurrently-live SIMD vectors.

Is looks like mergelocals will treat the live range as one instruction
if it's not closed: [st, st+1). Should we make it [st, +inf) instead? So
that we won't have similar errors in the future.

Also, I feel we really need to examine every "case types.TSTRUCT" or "if
t.Kind() == types.TSTRUCT" in the codebase correctly for SIMD types...

Change-Id: I2f4f4f36a890bd317d582cfa73a8f6a789382d91
Reviewed-on: https://go-review.googlesource.com/c/go/+/687775
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/liveness/plive.go

@@ -1534,6 +1534,9 @@ func isfat(t *types.Type) bool {
 			}
 			return true
 		case types.TSTRUCT:
+			if t.IsSIMD() {
+				return false
+			}
 			// Struct with 1 field, check if field is fat
 			if t.NumFields() == 1 {
 				return isfat(t.Field(0).Type)

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

@@ -850,13 +850,6 @@ func (f *Func) NewLocal(pos src.XPos, typ *types.Type) *ir.Name {
 // items larger than what CanSSA would allow (approximateky, we disallow things
 // marked as open defer slots so as to avoid complicating liveness
 // analysis.
-//
-// TODO: make SIMD variables mergible.
-//
-//		Right now this check excludes SIMD vars because sometimes two live SIMD
-//		vectors will be put into the same partition by mergelocals, we need to figure
-//		out why because these vectors are big and should be merged when possible.
-//	 Details in CL 687375.
 func IsMergeCandidate(n *ir.Name) bool {
 	if base.Debug.MergeLocals == 0 ||
 		base.Flag.N != 0 ||
@@ -864,7 +857,6 @@ func IsMergeCandidate(n *ir.Name) bool {
 		n.Type().Size() <= int64(3*types.PtrSize) ||
 		n.Addrtaken() ||
 		n.NonMergeable() ||
-		n.Type().IsSIMD() ||
 		n.OpenDeferSlot() {
 		return false
 	}