Auto merge of #133891 - nnethercote:MixedBitSet, r=Mark-Simulacrum

Introduce `MixedBitSet`

`ChunkedBitSet` is good at avoiding excessive memory usage for programs with very large functgions where dataflow bitsets have very large domain sizes. But it's overly heavyweight for small bitsets, because any non-empty `ChunkedBitSet` takes up at least 256 bytes.

This PR introduces `MixedBitSet`, which is a simple bitset that uses `BitSet` for small/medium bitsets and `ChunkedBitSet` for large bitsets. It's a speed and memory usage win.

r? `@Mark-Simulacrum`

Author: bors