pattern analysis: Use contiguous indices for enum variants
The main blocker to using the in-tree version of the `pattern_analysis` crate is that rustc requires enum indices to be contiguous because it uses `IndexVec`/`BitSet` for performance. Currently we swap these out for `FxHashMap`/`FxHashSet` when the `rustc` feature is off, but we can't do that if we use the in-tree crate.
This PR solves the problem by using contiguous indices on the r-a side too.
c82d168b79