gc: another shift bug

Fixes #1316.

R=ken2
CC=golang-dev
https://golang.org/cl/3575042

src/cmd/gc/const.c

@@ -101,7 +101,7 @@ convlit1(Node **np, Type *t, int explicit)
 		break;
 	case OLSH:
 	case ORSH:
-		convlit1(&n->left, t, explicit);
+		convlit1(&n->left, t, explicit && isideal(n->left->type));
 		t = n->left->type;
 		if(t != T && !isint[t->etype]) {
 			yyerror("invalid operation: %#N (shift of type %T)", n, t);

test/const3.go

@@ -26,4 +26,10 @@ func main() {
 		println("type info didn't propagate in const: got", s)
 		panic("fail")
 	}
+	x := uint(5)
+	y := float64(uint64(1)<<x)	// used to fail to compile
+	if y != 32 {
+		println("wrong y", y)
+		panic("fail")
+	}
 }