cmd/compile: support structural typing in unified IR

This CL updates unified IR to look at the structural type of a
composite literal type, rather than merely the underlying type, to
determine if it's a structure. This fixes a number of currently
failing regress test cases.

Updates #50833.

Change-Id: I11c040c77ec86c23e8ffefcf1ce1aed548687dc5
Reviewed-on: https://go-review.googlesource.com/c/go/+/381074


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>