Auto merge of #116152 - cjgillot:unchunck, r=nnethercote

Only use dense bitsets in dataflow analyses

When a dataflow state has the size close to the number of locals, we should prefer a dense bitset, like we already store locals in a dense vector.
Other occurrences of `ChunkedBitSet` need to be justified by the size of the dataflow state.

Author: bors