Auto merge of #111396 - vlad20012:reduce-Borrows-dataflow-bitset-size…

…, r=cjgillot

Reduce BitSet size used in `Borrows` dataflow analysis

It looks like it is not needed to multiply the number of borrows by 2. Bits greater than `self.borrow_set.len()` are never set in this bitset. This should decrease the memory usage by an epsilon.

compiler/rustc_borrowck/src/dataflow.rs

@@ -328,7 +328,7 @@ impl<'tcx> rustc_mir_dataflow::AnalysisDomain<'tcx> for Borrows<'_, 'tcx> {
 
     fn bottom_value(&self, _: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = nothing is reserved or activated yet;
-        BitSet::new_empty(self.borrow_set.len() * 2)
+        BitSet::new_empty(self.borrow_set.len())
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {