cmd/compile: replace len(Nodes.Slice()) with Nodes.Len()

Generated with eg:

func before(n gc.Nodes) int { return len(n.Slice()) }
func after(n gc.Nodes) int  { return n.Len() }

Change-Id: Ifdf01915e60069166afe96aa7b1d08720bf62fc5
Reviewed-on: https://go-review.googlesource.com/22420
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/cmd/compile/internal/gc/walk.go b/src/cmd/compile/internal/gc/walk.go
index 7e160bd..04ccfad 100644
--- a/src/cmd/compile/internal/gc/walk.go
+++ b/src/cmd/compile/internal/gc/walk.go
@@ -70,7 +70,7 @@
 	}
 
 	heapmoves()
-	if Debug['W'] != 0 && len(Curfn.Func.Enter.Slice()) > 0 {
+	if Debug['W'] != 0 && Curfn.Func.Enter.Len() > 0 {
 		s := fmt.Sprintf("enter %v", Curfn.Func.Nname.Sym)
 		dumplist(s, Curfn.Func.Enter)
 	}