cmd/8g: fix code generation of int64(0) == int64(0).

The code would violate the contract of cmp64.

Fixes #5002.

R=rsc, golang-dev
CC=golang-dev
https://golang.org/cl/7593043

src/cmd/8g/cgen.c

@@ -1055,7 +1055,7 @@ bgen(Node *n, int true, int likely, Prog *to)
 		}
 
 		if(is64(nr->type)) {
-			if(!nl->addable) {
+			if(!nl->addable || isconst(nl, CTINT)) {
 				tempname(&n1, nl->type);
 				cgen(nl, &n1);
 				nl = &n1;

test/fixedbugs/issue5002.go

+// build
+
+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 5002: 8g produces invalid CMPL $0, $0.
+// Used to fail at link time.
+
+package main
+
+func main() {
+	var y int64
+	if y%1 == 0 {
+	}
+}