[dev.regabi] cmd/compile: cleanup type switch typechecking

Address outstanding TODO, which simplifies subsequent CLs.

Now the compiler always type checks type-switch case clauses (like
gccgo), but it treats clause variables as broken if an appropriate
type cannot be determined for it (like go/types).

Passes toolstash-check.

Change-Id: Iedfe9cdf38c6865211e4b93391f1cf72c1bed136
Reviewed-on: https://go-review.googlesource.com/c/go/+/272648


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>