[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 <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

nawawi/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,golang/go,nawawi/go