Auto merge of #52250 - nnethercote:no-SparseBitMatrix, r=nikomatsakis

Speed up `SparseBitMatrix` use in `RegionValues`.

In practice, these matrices range from 10% to 90%+ full once they are
filled in, so the dense representation is better.

This reduces the runtime of Check Nll builds of `inflate` by 32%, and
several other benchmarks by 1--5%.

It also increases max-rss of `clap-rs` by 30% and a couple of others by
up to 5%, while decreasing max-rss of `coercions` by 14%. I think the
speed-ups justify the max-rss increases.

r? @nikomatsakis

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