gc: fix type switch error message for invalid cases.

Fixes #1606.

R=rsc
CC=golang-dev
https://golang.org/cl/4532045

src/cmd/gc/swt.c

@@ -867,8 +867,11 @@ typecheckswitch(Node *n)
 				case Etype:	// type switch
 					if(ll->n->op == OLITERAL && istype(ll->n->type, TNIL))
 						;
-					else if(ll->n->op != OTYPE && ll->n->type != T)
+					else if(ll->n->op != OTYPE && ll->n->type != T) {
 						yyerror("%#N is not a type", ll->n);
+						// reset to original type
+						ll->n = n->ntest->right;
+					}
 					break;
 				}
 			}

test/fixedbugs/bug340.go

+// errchk $G $D/$F.go
+
+// Copyright 2011 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 1606.
+
+package main
+
+func main() {
+	var x interface{}
+	switch t := x.(type) { // ERROR "0 is not a type"
+	case 0:
+		t.x = 1 // ERROR "type interface \{ \}"
+	}
+}