[release-branch.go1.5] cmd/compile: fix register allocation for == operator

The issue 12226 has been caused by the allocation of the same register
for the equality check of two byte values. The code in cgen.go freed the
register for the second operand before the allocation of the register
for the first operand.

Fixes #12226

Change-Id: Ie4dc33a488bd48a17f8ae9b497fd63c1ae390555
Reviewed-on: https://go-review.googlesource.com/13771Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/14227Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/cgen.go

@@ -2018,11 +2018,11 @@ func bgenx(n, res *Node, wantTrue bool, likely int, to *obj.Prog) {
 		Regalloc(&n2, nr.Type, nil)
 		Cgen(nr, &n2)
 		nr = &n2
-		Regfree(&n2)
 
 		Regalloc(&n1, nl.Type, nil)
 		Cgen(&tmp, &n1)
 		Regfree(&n1)
+		Regfree(&n2)
 	} else {
 		var n1 Node
 		if !nl.Addable && Ctxt.Arch.Thechar == '8' {

test/fixedbugs/issue12226.go

+// run
+
+// Copyright 2015 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.
+
+package main
+
+import "fmt"
+
+func main() {
+	if []byte("foo")[0] == []byte("b")[0] {
+		fmt.Println("BUG: \"foo\" and \"b\" appear to have the same first byte")
+	}
+}