don't track borrows for empty regions

Region inference can create borrows for an empty region if the borrow is
dead. In that case, there's no reason to track the borrow, but because
there's no such thing as an EndRegion(ReEmpty) these borrows used to live
for the entire function.

Fixes rust-lang#46161.

Author: arielb1

src/librustc_mir/dataflow/impls/borrows.rs

@@ -212,6 +212,12 @@ impl<'a, 'gcx, 'tcx> BitDenotation for Borrows<'a, 'gcx, 'tcx> {
             mir::StatementKind::Assign(_, ref rhs) => {
                 if let mir::Rvalue::Ref(region, _, ref place) = *rhs {
                     if is_unsafe_place(self.tcx, self.mir, place) { return; }
+                    if let RegionKind::ReEmpty = region {
+                        // If the borrowed value is dead, the region for it
+                        // can be empty. Don't track the borrow in that case.
+                        return
+                    }
+
                     let index = self.location_map.get(&location).unwrap_or_else(|| {
                         panic!("could not find BorrowIndex for location {:?}", location);
                     });

src/test/run-pass/issue-8860.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// compile-flags: -Z borrowck=compare
 
 static mut DROP: isize = 0;
 static mut DROP_S: isize = 0;