[dev.typeparams] cmd/compile/internal/types2: replace optype() with under() in variou...

[dev.typeparams] cmd/compile/internal/types2: replace optype() with under() in various cases (cleanup)

This makes the behavior for type parameter operands explicit
in those cases.

Change-Id: I38438af67de4432f1a691dc4947e4576445f031b
Reviewed-on: https://go-review.googlesource.com/c/go/+/332555
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>