cmd/gc: avoid passing unevaluated constant expressions to backends.

Backends do not exactly expect receiving binary operators with
constant operands or use workarounds to move them to
register/stack in order to handle them.

Fixes #5841.

R=golang-dev, daniel.morsing, rsc
CC=golang-dev
https://golang.org/cl/11107044

src/cmd/gc/walk.c

@@ -1379,6 +1379,13 @@ walkexpr(Node **np, NodeList **init)
 	fatal("missing switch %O", n->op);
 
 ret:
+	// Expressions that are constant at run time but not
+	// considered const by the language spec are not turned into
+	// constants until walk. For example, if n is y%1 == 0, the
+	// walk of y%1 may have replaced it by 0.
+	// Check whether n with its updated args is itself now a constant.
+	evconst(n);
+
 	ullmancalc(n);
 
 	if(debug['w'] && n != N)

test/fixedbugs/issue5841.go

@@ -0,0 +1,16 @@
+// 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 5841: 8g produces invalid CMPL $0, $0.
+// Similar to issue 5002, used to fail at link time.
+
+package main
+
+func main() {
+	var y int
+	if y%1 == 0 {
+	}
+}