cmd/gc: fix codegen reordering for expressions involving && and ||

Fixes #2821.

R=rsc
CC=golang-dev
https://golang.org/cl/5606061

src/cmd/gc/subr.c

@@ -1670,6 +1670,11 @@ ullmancalc(Node *n)
 	if(n == N)
 		return;
 
+	if(n->ninit != nil) {
+		ul = UINF;
+		goto out;
+	}
+
 	switch(n->op) {
 	case OREGISTER:
 	case OLITERAL:
@@ -3577,4 +3582,5 @@ addinit(Node **np, NodeList *init)
 		break;
 	}
 	n->ninit = concat(init, n->ninit);
+	n->ullman = UINF;
 }

src/cmd/gc/walk.c

@@ -1203,10 +1203,11 @@ walkexpr(Node **np, NodeList **init)
 	fatal("missing switch %O", n->op);
 
 ret:
+	ullmancalc(n);
+
 	if(debug['w'] && n != N)
 		dump("walk", n);
 
-	ullmancalc(n);
 	lineno = lno;
 	*np = n;
 }

test/fixedbugs/bug406.go

@@ -0,0 +1,25 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out || echo "Bug406"
+
+// Copyright 2012 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 2821
+package main
+
+type matrix struct {
+	e []int
+}
+
+func (a matrix) equal() bool {
+	for _ = range a.e {
+	}
+	return true
+}
+
+func main() {
+	var a matrix
+	var i interface{}
+	i = true && a.equal()
+	_ = i
+}