cmd/compile: support reading union type for compiler backend in unified IR

Fixes #52124

Change-Id: I5749822d41d8e51f476bceb277b1d2cf7350dcc3
Reviewed-on: https://go-review.googlesource.com/c/go/+/397874


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>