cmd/compile: allow unsafe.Pointer(nil) as static data

Fixes #16306

Change-Id: If8e2f411fe9a5a5c198f10765fee7261ba8feaf2
Reviewed-on: https://go-review.googlesource.com/24836
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -1435,7 +1435,7 @@ func genAsInitNoCheck(n *Node, reportOnly bool) bool {
 
 	case TBOOL, TINT8, TUINT8, TINT16, TUINT16,
 		TINT32, TUINT32, TINT64, TUINT64,
-		TINT, TUINT, TUINTPTR,
+		TINT, TUINT, TUINTPTR, TUNSAFEPTR,
 		TPTR32, TPTR64,
 		TFLOAT32, TFLOAT64:
 		if !reportOnly {

test/fixedbugs/issue16306.go

+// 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 main
+
+import "unsafe"
+
+var x = unsafe.Pointer(uintptr(0))
+
+func main() {
+	_ = map[unsafe.Pointer]int{unsafe.Pointer(uintptr(0)): 0}
+}