use precalculated dominators in explain_borrow

This looks like the only place calculating dominators from the MIR body every time instead of using the ones stored on the `MirBorrowckCtxt`. For example, in rust-lang#65131 a big generated function with a number of borrowck errors takes a few hours(!) recalculating the dominators while explaining the errors.

I don't know enough about this part of rustc codebase to know for sure that this change is correct, but no tests seem to fail as a result of this change in local testing.

Author: tanriol

src/librustc_mir/borrow_check/nll/explain_borrow/mod.rs

@@ -457,7 +457,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     /// True if an edge `source -> target` is a backedge -- in other words, if the target
     /// dominates the source.
     fn is_back_edge(&self, source: Location, target: Location) -> bool {
-        target.dominates(source, &self.body.dominators())
+        target.dominates(source, &self.dominators)
     }
 
     /// Determine how the borrow was later used.