ssa: compile go string constant without alloca

cl/_testgo/multiret/in.go

@@ -0,0 +1,12 @@
+package main
+
+var a int = 1
+
+func foo(f float64) (int, float64) {
+	return a, f
+}
+
+func main() {
+	i, f := foo(2.0)
+	println(i, f)
+}