cmd/compile: use Structure() to get single underlying type of typeparam.
Use types2.Structure() to get single underlying type of typeparams, to
handle some unusual cases where a type param is constrained to a single
underlying struct or map type.
Fixes #48538
Change-Id: I289fb7b31d489f7586f2b04aeb1df74e15a9f965
Reviewed-on: https://go-review.googlesource.com/c/go/+/359335
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>