gc: fix struct and array comparisons for new bool rules

The two optimizations for small structs and arrays
were missing the implicit cast from ideal bool.

Fixes #3351.

R=rsc, lvd
CC=golang-dev
https://golang.org/cl/5848062

src/cmd/gc/walk.c

@@ -2514,6 +2514,7 @@ walkcompare(Node **np, NodeList **init)
 			expr = nodbool(n->op == OEQ);
 		typecheck(&expr, Erv);
 		walkexpr(&expr, init);
+		expr->type = n->type;
 		*np = expr;
 		return;
 	}
@@ -2534,6 +2535,7 @@ walkcompare(Node **np, NodeList **init)
 			expr = nodbool(n->op == OEQ);
 		typecheck(&expr, Erv);
 		walkexpr(&expr, init);
+		expr->type = n->type;
 		*np = expr;
 		return;
 	}

test/fixedbugs/bug427.go

@@ -0,0 +1,39 @@
+// compile
+
+// 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.
+
+// http://code.google.com/p/go/issues/detail?id=3351
+
+package main
+
+// struct with four fields of basic type
+type S struct {a, b, c, d int}
+
+// struct with five fields of basic type
+type T struct {a, b, c, d, e int}
+
+// array with four elements
+type A [4]int
+
+// array with five elements
+type B [5]int
+
+func main() {
+	var i interface{}
+
+	var s1, s2 S
+	i = s1 == s2
+
+	var t1, t2 T
+	i = t1 == t2
+
+	var a1, a2 A
+	i = a1 == a2
+
+	var b1, b2 B
+	i = b1 == b2
+
+	_ = i
+}