[dev.typeparams] cmd/compile/internal/ir: more useful Fatalfs

This CL just adds some additional details to existing Fatalf messages
that make them more useful for identifying what went wrong.

Change-Id: Icba0d943ccfb1b810a1ede0977cc8cf22b2afde5
Reviewed-on: https://go-review.googlesource.com/c/go/+/320612


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>