cmd/compile: fix another invalid switch case panic

Very similar fix to the one made in golang.org/cl/65655. This time it's
for switches on interface values, as we look for duplicates in a
different manner to keep types in mind.

As before, add a small regression test.

Updates #22001.
Fixes #22063.

Change-Id: I9a55d08999aeca262ad276b4649b51848a627b02
Reviewed-on: https://go-review.googlesource.com/66450
Run-TryBot: Daniel Martí <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

Author: mvdan

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

@@ -640,6 +640,11 @@ func checkDupExprCases(exprname *Node, clauses []*Node) {
 			if ct := consttype(n); ct < 0 || ct == CTBOOL {
 				continue
 			}
+			// If the value has no type, we have
+			// already printed an error about it.
+			if n.Type == nil {
+				continue
+			}
 			tv := typeVal{
 				typ: n.Type.LongString(),
 				val: n.Val().Interface(),

test/fixedbugs/issue22063.go

@@ -0,0 +1,17 @@
+// errorcheck
+
+// Copyright 2017 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.
+
+// Issue 22063: panic on interface switch case with invalid name
+
+package p
+
+const X = Wrong(0) // ERROR "undefined: Wrong"
+
+func _() {
+	switch interface{}(nil) {
+	case X:
+	}
+}