cmd/compile: separate type and expression switch typechecking

While superficially type and expression switch handling seem similar
and that it would be worthwhile to unify typechecking them, it turns
out they're actually different enough that separately handling them is
fewer lines of code and easier to understand as well.

Passes toolstash-check.

Change-Id: I357d6912dd580639b6001bccdb2e227ed83c6fe9
Reviewed-on: https://go-review.googlesource.com/c/go/+/194566
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>