cmd/compile: fix ICE in reporting of invalid recursive types

cmd/compile: fix ICE in reporting of invalid recursive types

asNode(t.Nod).Name.Param will be nil for builtin types (i.e., the
universal predeclared types and unsafe.Pointer). These types can't be
part of a cycle anyway, so we can just skip them.

Fixes #42075.

Change-Id: Ic7a44de65c6bfd16936545dee25e36de8850acf3
Reviewed-on: https://go-review.googlesource.com/c/go/+/263717
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>