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

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>