[dev.typeparams] cmd/compile: fail early on unexpected types2.Invalid
In unified IR, fail right away if we find a types2.Invalid while
writing out the package. This provides a clearer error message for
https://github.com/golang/go/issues/25838#issuecomment-
448746670.
Updates #25838.
Change-Id: I6902fdd891fc31bbb832b6fdba00eca301282409
Reviewed-on: https://go-review.googlesource.com/c/go/+/338973
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>