cmd/compile: skip looking for OCLOSURE nodes in xtop

xtop holds package's top-level declaration statements, but OCLOSURE
only appears in expression contexts. xtop will instead hold the
synthetic ODCLFUNC representing OCLOSURE's function body.

This CL makes the loop consistent with the later phases that only look
for ODCLFUNC nodes in xtop.

Passes toolstash-check.

Change-Id: I852a10ef1bf75bb3351e3da0357ca8b2e26aec6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/255340


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>