[dev.regabi] cmd/compile: arrange for walkstmt, walkexpr, to return from switch cases

[dev.regabi] cmd/compile: arrange for walkstmt, walkexpr, to return from switch cases

Ending them in a returning switch makes it safe for each case
to do an appropriate type assertion.

Passes buildall w/ toolstash -cmp.

Change-Id: I55d8f0a555006104164d84d27822aa8c5ad68515
Reviewed-on: https://go-review.googlesource.com/c/go/+/275882
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>