gc: remove now redundant typecheck of ->ninit on switches.

Fixes #2576.

R=rsc
CC=golang-dev
https://golang.org/cl/5498105

src/cmd/gc/swt.c

@@ -792,7 +792,6 @@ walkswitch(Node *sw)
 	 * cases have OGOTO into statements.
 	 * both have inserted OBREAK statements
 	 */
-	walkstmtlist(sw->ninit);
 	if(sw->ntest == N) {
 		sw->ntest = nodbool(1);
 		typecheck(&sw->ntest, Erv);

test/fixedbugs/bug391.go

+// $G $D/$F.go || echo "Issue2576"
+
+// 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 2576
+package bug
+
+type T struct { a int }
+
+func f(t T) {
+        switch _, _ = t.a, t.a; {}
+}
\ No newline at end of file