cmd/compile: fix TypeDefn to deal with node with no Ntype set
Adjust TypeDefn(), which is used by reportTypeLoop(), to work for nodes
with no Ntype set (which are all nodes in -G=3 mode). Normally,
reportTypeLoop() would not be called, because the types2 typechecker
would have already caught it. This is a possible way to report an
unusual type loop involving type params, which is not being caught by
the types2 type checker.
Updates #48962
Change-Id: I55edee46026eece2e8647c5b5b4d8dfb39eeb5f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/361398
Trust: Dan Scales <danscales@google.com> Reviewed-by: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Robert Griesemer <gri@golang.org>