Fix ICE when assigning references to a static mut with NLL

is_unsafe_place only filters out statics in the rhs, not the lhs. Since
it's possible to reach that 'Place::Static', we handle statics the same
way as we do locals.

Fixes #47789

src/librustc_mir/dataflow/impls/borrows.rs

@@ -396,8 +396,7 @@ impl<'a, 'gcx, 'tcx> Borrows<'a, 'gcx, 'tcx> {
                         // Issue #46746: Two-phase borrows handles
                         // stmts of form `Tmp = &mut Borrow` ...
                         match lhs {
-                            Place::Local(..) => {} // okay
-                            Place::Static(..) => unreachable!(), // (filtered by is_unsafe_place)
+                            Place::Local(..) | Place::Static(..) => {} // okay
                             Place::Projection(..) => {
                                 // ... can assign into projections,
                                 // e.g. `box (&mut _)`. Current

src/test/run-pass/issue-47789.rs

@@ -0,0 +1,20 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+
+#![feature(nll)]
+
+static mut x: &'static u32 = &0;
+
+fn foo() {
+    unsafe { x = &1; }
+}
+
+fn main() { }