Rollup merge of #74169 - ecstatic-morse:dataflow-unreachable, r=pnkfelix

Rollup merge of #74169 - ecstatic-morse:dataflow-unreachable, r=pnkfelix

Stop processing unreachable blocks when solving dataflow

...instead we `debug_assert` that the user is not checking the dataflow state for an unreachable block. This resolves a FIXME in the dataflow engine. The old behavior was an artifact of the previous dataflow framework. Things should run a tiny bit faster now, but I suspect not enough to show up in benchmarks. AFAIK, only the generator transform runs dataflow on MIR with unreachable basic blocks.

This PR also adds some utility methods to `mir::traversal`.

r? @pnkfelix