[dev.regabi] cmd/compile: refactor type/value assertions

Small refactoring to make subsequent CLs clearer.

Passes toolstash-check.

Change-Id: I1a6ae599f491220d44aaabae0b7bed4aff46ee92
Reviewed-on: https://go-review.googlesource.com/c/go/+/272651


Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>