cmd/internal/gc: explicitly set zero bool Val

This trivial change is a prerequisite to
converting Val.U to an interface{}.

No functional changes. Passes toolstash -cmp.

Change-Id: I17ff036f68d29a9ed0097a8b23ae1c91e6ce8c21
Reviewed-on: https://go-review.googlesource.com/10058Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/gc/gen.go

@@ -348,6 +348,7 @@ func Clearslim(n *Node) {
 
 	case TBOOL:
 		z.Val.Ctype = CTBOOL
+		z.Val.U.Bval = false
 
 	case TINT8,
 		TINT16,