cmd/compile: fix invalid switch case value panic

This is a regression introduced by myself in golang.org/cl/41852,
confirmed by the program that reproduces the crash that can be seen in
the added test.

Fixes #21988.

Change-Id: I18d5b2b3de63ced84db705b18490b00b16b59e02
Reviewed-on: https://go-review.googlesource.com/65655
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -610,6 +610,11 @@ 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
+				}
 
 				val := n.Val().Interface()
 				prev, dup := seen[val]

test/fixedbugs/issue21988.go

+// 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 21988: panic on switch case with invalid value
+
+package p
+
+const X = Wrong(0) // ERROR "undefined: Wrong"
+
+func _() {
+	switch 0 {
+	case X:
+	}
+}