Optimize dataflow-const-prop place-tracking infra
Optimization opportunities found while investigating https://github.com/rust-lang/rust/pull/110719
Computing places breadth-first ensures that we create short projections before deep projections, since the former are more likely to be propagated.
The most relevant is the pre-computation of flooded places. Callgrind showed `flood_*` methods and especially `preorder_preinvoke` were especially hot. This PR attempts to pre-compute the set of `ValueIndex` that `preorder_invoke` would visit.
Using this information, we make some `PlaceIndex` inaccessible when they contain no `ValueIndex`, allowing to skip computations for those places.
cc `@jachris` as original author
9a767b6b9e