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>

src/cmd/compile/internal/gc/align.go

@@ -199,7 +199,7 @@ func findTypeLoop(t *types.Type, path *[]*types.Type) bool {
 		}
 
 		*path = append(*path, t)
-		if findTypeLoop(asNode(t.Nod).Name.Param.Ntype.Type, path) {
+		if p := asNode(t.Nod).Name.Param; p != nil && findTypeLoop(p.Ntype.Type, path) {
 			return true
 		}
 		*path = (*path)[:len(*path)-1]

test/fixedbugs/issue42075.go

@@ -0,0 +1,16 @@
+// errorcheck
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+type T struct { // ERROR "recursive type"
+	x int
+	p unsafe.Pointer
+
+	f T
+}