[dev.typeparams] go/types: replace optype() with under() in various cases (cleanup)

[dev.typeparams] go/types: replace optype() with under() in various cases (cleanup)

This is a port of CL 332555 to go/types.

Change-Id: I9b26bba8b605f5bbbd8f0f81fd6651a4a3ff4b57
Reviewed-on: https://go-review.googlesource.com/c/go/+/335111
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>