cmd/compile/internal/types2: make sure we are safe for nil in underIs

Reviewed all uses of underIs (global function and method) and made
sure we are ok with a nil incoming argument (indicating a type set
with no specific types).

Added a couple of checks where we didn't have them (and somehow
didn't run into a problem yet).

Change-Id: Ifde45a3a80ddf2b1a19c83f79258ad8207dfb09f
Reviewed-on: https://go-review.googlesource.com/c/go/+/363658


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>