fast path: compute `MaybeInitializedPlaces` lazily

Only drop-liveness checks for maybe-initializedness of move paths, and
it does so only for the relevant live locals that have drop points.

This adds a fast path by computing this dataflow analysis only when checking for such
initializedness. This avoids this expensive computation for the common
case.

For example, it avoids computing initializedness for 20K locals in the
`cranelift-codegen` benchmark, it has 7K relevant live locals but none
with drop points. That saves 900ms on end-to-end compilation times.

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