[dev.regabi] cmd/compile: remove walkReturn "common case" path

After the previous two optimization CLs, this code path now generates
the same code as ascompatee does anyway. So just use that and remove
some redundant code.

Passes toolstash -cmp.

Change-Id: I5e2e5c6dbea64d8e91abe0f2cf51aa5bb86576d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/281154


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