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.

Author: bors