Auto merge of #110820 - cjgillot:faster-dcp, r=oli-obk

Optimize dataflow-const-prop place-tracking infra

Optimization opportunities found while investigating rust-lang/rust#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

Author: bors