cmd/compile: remove zeroing after newobject

The Zero op right after newobject has been removed. But this rule
does not cover Store of constant zero (for SSA-able types). Add
rules to cover Store op as well.

Updates #19027.

Change-Id: I5d2b62eeca0aa9ce8dc7205b264b779de01c660b
Reviewed-on: https://go-review.googlesource.com/36836
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/rewrite.go

@@ -541,3 +541,13 @@ func experiment(f *Func) bool {
 	}
 	return r
 }
+
+func isConstZero(v *Value) bool {
+	switch v.Op {
+	case OpConstNil:
+		return true
+	case OpConst64, OpConst32, OpConst16, OpConst8, OpConstBool, OpConst32F, OpConst64F:
+		return v.AuxInt == 0
+	}
+	return false
+}