[dev.regabi] cmd/compile: refactor closure var setup/teardown

Creating closure vars is subtle and is also needed in both CL 281932
and CL 283112, so refactor out a common implementation that can be
used in all 3 places.

Passes toolstash -cmp.

Change-Id: Ib993eb90c895b52759bfbfbaad88921e391b0b4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/283194


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
Trust: Dan Scales <danscales@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>