[dev.regabi] cmd/compile: transform closures during walk

We used to transform directly called closures in a separate pass
before walk, because we couldn't guarantee whether we'd see the
closure call or the closure itself first. As of the last CL, this
ordering is always guaranteed, so we can rewrite calls and the closure
at the same time.

Change-Id: Ia6f4d504c24795e41500108589b53395d301123b
Reviewed-on: https://go-review.googlesource.com/c/go/+/283315


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>