cmd/internal/gc: more Node cleanups

More Node cleanups, these ones touch go.y.

- convert Node.Implicit to bool
- convert Node.Used to bool

Change-Id: I85c7ff9e66cee7122b560adedc995166c874f2f2
Reviewed-on: https://go-review.googlesource.com/7124
Reviewed-by: Keith Randall <khr@golang.org>
diff --git a/src/cmd/internal/gc/gen.go b/src/cmd/internal/gc/gen.go
index 7089b9ce..70c218e 100644
--- a/src/cmd/internal/gc/gen.go
+++ b/src/cmd/internal/gc/gen.go
@@ -613,7 +613,7 @@
 func temp(t *Type) *Node {
 	n := Nod(OXXX, nil, nil)
 	Tempname(n, t)
-	n.Sym.Def.Used = 1
+	n.Sym.Def.Used = true
 	return n.Orig
 }