gc: handle _ = <-c in select.

Fixes #238.

R=ken2
https://golang.org/cl/163098

src/cmd/gc/select.c

@@ -118,7 +118,7 @@ walkselect(Node *sel)
 			else {
 				// introduce temporary until we're sure this will succeed.
 				tmp = nod(OXXX, N, N);
-				tempname(tmp, n->left->type);
+				tempname(tmp, n->right->type->type);
 				a = nod(OADDR, tmp, N);
 			}
 			// selectrecv(sel *byte, hchan *chan any, elem *any) (selected bool);

test/fixedbugs/bug225.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2009 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.
+
+package main
+
+func main() {	
+	c := make(chan bool, 1);
+	select {
+	case _ = <-c:
+		panic("BUG: recv should not");
+	default:
+	}
+	c <- true;
+	select {
+	case _ = <-c:
+	default:
+		panic("BUG: recv should");
+	}
+}

test/golden.out

@@ -142,10 +142,6 @@ panic PC=xxx
 
 == bugs/
 
-=========== bugs/bug218.go
-<epoch>: fatal error: dowidth: unknown type: blank
-BUG: bug218
-
 =========== bugs/bug219.go
 bugs/bug219.go:16: syntax error near if
 BUG: bug219