cmd/internal/obj: update Bool2int to the form optimized by the compiler

As of https://golang.org/cl/22711 the compiler optimizes this form.

Updates #6011

Change-Id: Ibc6c529dfa24d42f4aab78ebd6722e1d72cb6038
Reviewed-on: https://go-review.googlesource.com/27395Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/util.go

@@ -504,8 +504,13 @@ var Anames = []string{
 }
 
 func Bool2int(b bool) int {
+	// The compiler currently only optimizes this form.
+	// See issue 6011.
+	var i int
 	if b {
-		return 1
+		i = 1
+	} else {
+		i = 0
 	}
-	return 0
+	return i
 }