gc: do not compile switch on interface values into a binary search.

Fixes #2672.

R=golang-dev, lvd
CC=golang-dev, remy
https://golang.org/cl/5543058

src/cmd/gc/swt.c

@@ -540,7 +540,7 @@ loop:
 	}
 
 	// deal with the variables one-at-a-time
-	if(c0->type != Texprconst) {
+	if(!okforcmp[t->etype] || c0->type != Texprconst) {
 		a = exprbsw(c0, 1, arg);
 		cas = list(cas, a);
 		c0 = c0->link;

test/fixedbugs/bug393.go

+// $G $D/$F.go || echo BUG: bug393
+
+// Copyright 2012 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 2672
+// was trying binary search with an interface type
+
+package main
+
+func f(x interface{}) int {
+	switch x {
+	case 1:
+		return 1
+	case 2:
+		return 2
+	case 3:
+		return 3
+	case 4:
+		return 4
+	case "5":
+		return 5
+	case "6":
+		return 6
+	default:
+		return 7
+	}
+	panic("switch")
+}