This is just replicating the previous algorithm, but taking advantage of the
bitset structures to optimize into tighter and better optimized loops.
Particularly advantageous on enormous MIR blocks, which are relatively rare in
practice.
The PR had some unforseen perf regressions that are not as easy to find.
Revert the PR for now.
This reverts commit 6ae8912a3e7d2c4c775024f58a7ba4b1aedc4073, reversing
changes made to 86d6d2b7389fe1b339402c1798edae8b695fc9ef.
With the arrival of min const generics, many alt-vec libraries have
updated to use it in some way and arrayvec is no exception. Use the
latest with minor refactoring.
Also, rustc_workspace_hack is the only user of smallvec 0.6 in the
entire tree, so drop it.
A few small cleanups to `BitSet` and friends:
- Overload `clone_from` for `BitSet`.
- Improve `Debug` represenation of `HybridBitSet`.
- Make `HybridBitSet::domain_size` public.
- Don't require `T: Idx` at the type level. The `Idx` bound is still on
most `BitSet` methods, but like `HashMap`, it doesn't need to be
satisfied for the type to exist.
