Tweak MaybeBorrowedLocals::transfer_function usage.

In `MaybeRequiresStorage::apply_before_statement_effect`, call
`transfer_function` directly, as is already done in
`MaybeRequiresStorage::apply_before_terminator_effect`. This makes it clear
that the operation doesn't rely on the `MaybeBorrowedLocals` results.

Author: nnethercote

compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs

@@ -135,7 +135,7 @@ impl<'tcx> Analysis<'tcx> for MaybeRequiresStorage<'_, 'tcx> {
         loc: Location,
     ) {
         // If a place is borrowed in a statement, it needs storage for that statement.
-        self.borrowed_locals.mut_analysis().apply_statement_effect(trans, stmt, loc);
+        MaybeBorrowedLocals::transfer_function(trans).visit_statement(stmt, loc);
 
         match &stmt.kind {
             StatementKind::StorageDead(l) => trans.kill(*l),