Compress `Liveness` data some more.

Profiling shows that the `(reader, writer, used)` triples used by
liveness analysis almost always have invalid `reader` and `writer`
fields. We can take advantage of this knowledge to use a compressed
representation for them, falling back to a secondary table for the
uncommon cases.

This change reduces instruction counts on numerous benchmarks, the best
by 16%. It also reduces max-rss on numerous benchmarks, the best by 38%.

The patch also renames these triples from `Users` to `RWU`, because it's
confusing having a type whose name is plural and then used within
vectors whose names are also plural.

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