cmd/compile: never CSE two memories

It never makes sense to CSE two ops that generate memory.
We might as well start those ops off in their own partition.

Fixes #15520

Change-Id: I0091ed51640f2c10cd0117f290b034dde7a86721
Reviewed-on: https://go-review.googlesource.com/22741Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/subr.go

@@ -1118,10 +1118,6 @@ func syslook(name string) *Node {
 	return s.Def
 }
 
-func (s *Sym) IsRuntimeCall(name string) bool {
-	return s.Pkg == Runtimepkg && s.Name == name
-}
-
 // typehash computes a hash value for type t to use in type switch
 // statements.
 func typehash(t *Type) uint32 {

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

@@ -116,12 +116,6 @@ type GCNode interface {
 	String() string
 }
 
-// GCSym is an interface that *gc.Sym implements.
-// Using *gc.Sym directly would lead to import cycles.
-type GCSym interface {
-	IsRuntimeCall(name string) bool
-}
-
 // NewConfig returns a new configuration object for the given architecture.
 func NewConfig(arch string, fe Frontend, ctxt *obj.Link, optimize bool) *Config {
 	c := &Config{arch: arch, fe: fe}

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

@@ -257,13 +257,10 @@ func cmpVal(v, w *Value, auxIDs auxmap, depth int) Cmp {
 	if v.Op == OpPhi && v.Block != w.Block {
 		return lt2Cmp(v.Block.ID < w.Block.ID)
 	}
-
-	switch v.Op {
-	case OpStaticCall, OpAMD64CALLstatic, OpARMCALLstatic:
-		sym := v.Aux.(GCSym)
-		if sym.IsRuntimeCall("newobject") {
-			return lt2Cmp(v.ID < w.ID)
-		}
+	if v.Type.IsMemory() {
+		// We will never be able to CSE two values
+		// that generate memory.
+		return lt2Cmp(v.ID < w.ID)
 	}
 
 	if tc := v.Type.Compare(w.Type); tc != CMPeq {