cmd/gc: fix LEAQ $0, SI bug

Cannot take effective address of constant.

Fixes #3670.

R=ken2
CC=golang-dev
https://golang.org/cl/6299055

src/cmd/gc/gen.c

@@ -647,6 +647,9 @@ cgen_as(Node *nl, Node *nr)
 		dump("cgen_as = ", nr);
 	}
 
+	while(nr != N && nr->op == OCONVNOP)
+		nr = nr->left;
+
 	if(nl == N || isblank(nl)) {
 		cgen_discard(nr);
 		return;

test/fixedbugs/bug444.go

+// run
+
+// 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.
+
+// The no-op conversion here used to confuse the compiler
+// into doing a load-effective-address of nil.
+
+package main
+
+import "reflect"
+
+type T interface {}
+
+func main() {
+        reflect.TypeOf(nil)
+        reflect.TypeOf(T(nil)) // used to fail
+}