ICE when assigning to a static mut

[nikomatsakis]

This code assigning to a static mut causes an ICE when run with feature(nll):

#![feature(nll)]

static y: u32 = 22;
static mut x: &'static u32 = &y;

fn foo() {
    unsafe { x = &y; }
}

fn main() { }

On the playground, I get this:

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.25.0-nightly (9fd7da904 2018-01-25) running on x86_64-unknown-linux-gnu

thread 'rustc' panicked at 'internal error: entered unreachable code', librustc_mir/dataflow/impls/borrows.rs:400:50
note: Run with `RUST_BACKTRACE=1` for a backtrace.

[ritiek]

cc @pnkfelix who wrote related code in #46887.

[Aaron1011]

I'd like to work on this.

[nikomatsakis]

@Aaron1011 great =)