cmd/gc: add missing typecheck for walk-generated constants.

Fixes #6131.

R=golang-dev, daniel.morsing, rsc
CC=golang-dev
https://golang.org/cl/12800045

src/cmd/gc/walk.c

@@ -1387,6 +1387,8 @@ ret:
 	// walk of y%1 may have replaced it by 0.
 	// Check whether n with its updated args is itself now a constant.
 	evconst(n);
+	if(n->op == OLITERAL)
+		typecheck(&n, Erv);
 
 	ullmancalc(n);
 

test/fixedbugs/issue6131.go

@@ -0,0 +1,20 @@
+// compile
+
+// 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 6131: missing typecheck after reducing
+// n%1 == 0 to a constant value.
+
+package main
+
+func isGood(n int) bool {
+	return n%1 == 0
+}
+
+func main() {
+	if !isGood(256) {
+		panic("!isGood")
+	}
+}