[dev.typealias] cmd/compile: various minor cleanups

Also: Don't allow type pragmas with type alias declarations.

For #18130.

Change-Id: Ie54ea5fefcd677ad87ced03466bbfd783771e974
Reviewed-on: https://go-review.googlesource.com/35102


Reviewed-by: Matthew Dempsky <mdempsky@google.com>