cmd/compile: fix static-initialization compilation failure

Fixes #13263.

Change-Id: Ie1cafc62b6bfe6c5381c35d9a95563267b4cc9b0
Reviewed-on: https://go-review.googlesource.com/33970
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/sinit.go

@@ -295,7 +295,9 @@ func staticcopy(l *Node, r *Node, out *[]*Node) bool {
 		if staticcopy(l, r, out) {
 			return true
 		}
-		*out = append(*out, nod(OAS, l, r))
+		// We may have skipped past one or more OCONVNOPs, so
+		// use conv to ensure r is assignable to l (#13263).
+		*out = append(*out, nod(OAS, l, conv(r, l.Type)))
 		return true
 
 	case OLITERAL:

test/fixedbugs/issue13263.go

@@ -0,0 +1,15 @@
+// compile
+
+// Copyright 2016 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 b
+
+var (
+	x uint
+	y = x
+	z = uintptr(y)
+	a = uint32(y)
+	b = uint64(y)
+)