cmd/compile: replace Ctype switches with type switches

cmd/compile: replace Ctype switches with type switches

Instead of switching on Ctype (which internally uses a type switch)
and then scattering lots of type assertions throughout the CTFOO case
clauses, just use type switches directly on the underlying constant
value.

Passes toolstash/buildall.

Change-Id: I9bc172cc67e5f391cddc15539907883b4010689e
Reviewed-on: https://go-review.googlesource.com/22384
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>