cmd/compile: remove two out-of-phase calls to walk

cmd/compile: remove two out-of-phase calls to walk

All calls to walkstmt/walkexpr/etc should be rooted from funccompile,
whereas transformclosure and fninit are called by main.

Passes toolstash-check.

Change-Id: Ic880e2d2d83af09618ce4daa8e7716f6b389e53e
Reviewed-on: https://go-review.googlesource.com/99418
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>