cmd/compile: fix optimization of Zero newobject on amd64p32

On amd64p32, PtrSize and RegSize don't agree, and function return
value is aligned with RegSize. Fix this rule. Other architectures
are not affected, where PtrSize and RegSize are the same.

Change-Id: If187d3dfde3dc3b931b8e97db5eeff49a781551b
Reviewed-on: https://go-review.googlesource.com/37720
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -1386,9 +1386,9 @@
 
 // recognize runtime.newobject and don't Zero/Nilcheck it
 (Zero (Load (OffPtr [c] (SP)) mem) mem)
-        && mem.Op == OpStaticCall
+	&& mem.Op == OpStaticCall
 	&& isSameSym(mem.Aux, "runtime.newobject")
-	&& c == config.ctxt.FixedFrameSize() + config.PtrSize // offset of return value
+	&& c == config.ctxt.FixedFrameSize() + config.RegSize // offset of return value
 	-> mem
 // nil checks just need to rewrite to something useless.
 // they will be deadcode eliminated soon afterwards.

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

@@ -17210,7 +17210,7 @@ func rewriteValuegeneric_OpZero(v *Value, config *Config) bool {
 	b := v.Block
 	_ = b
 	// match: (Zero (Load (OffPtr [c] (SP)) mem) mem)
-	// cond: mem.Op == OpStaticCall 	&& isSameSym(mem.Aux, "runtime.newobject") 	&& c == config.ctxt.FixedFrameSize() + config.PtrSize
+	// cond: mem.Op == OpStaticCall 	&& isSameSym(mem.Aux, "runtime.newobject") 	&& c == config.ctxt.FixedFrameSize() + config.RegSize
 	// result: mem
 	for {
 		v_0 := v.Args[0]
@@ -17230,7 +17230,7 @@ func rewriteValuegeneric_OpZero(v *Value, config *Config) bool {
 		if mem != v.Args[1] {
 			break
 		}
-		if !(mem.Op == OpStaticCall && isSameSym(mem.Aux, "runtime.newobject") && c == config.ctxt.FixedFrameSize()+config.PtrSize) {
+		if !(mem.Op == OpStaticCall && isSameSym(mem.Aux, "runtime.newobject") && c == config.ctxt.FixedFrameSize()+config.RegSize) {
 			break
 		}
 		v.reset(OpCopy)