cmd/internal/gc: make Node.Needzero a bool

Node.Needzero only has two values and acts as a bool, so make it a bool.

Change-Id: Ica46e5ebafbe478017ea52ce6bb335f404059677
Reviewed-on: https://go-review.googlesource.com/6800
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/gc/pgen.go

@@ -211,8 +211,8 @@ func cmpstackvar(a *Node, b *Node) int {
 		return bp - ap
 	}
 
-	ap = int(a.Needzero)
-	bp = int(b.Needzero)
+	ap = bool2int(a.Needzero)
+	bp = bool2int(b.Needzero)
 	if ap != bp {
 		return bp - ap
 	}