Handle irreducible control flow in branch simplification

[fitzgen]

We previously removed a block from the CFG if it was not marked as reachable by the time the egraph pass visited it. The pass's traversal is both (1) a depth-first pre-order dominator traversal, and (2) a reverse post-order CFG traversal. This traversal visits all of a block's non-back edge predecessors before visiting the block itself. For reducible control flow, this is all that is necessary because we've already visited every back edge's target block already.

However, for irreducible control flow, blocks can be reachable only through back edges, and so the traversal's property alone was not sufficient. (The EgraphBlockIter's proof is still correct, at least to the best of my knowledge since we haven't mechanically proven it, but the implicit assumption that its proven property is sufficient for our reachability-based block removal is incorrect in the face of irreducible control flow.)

This commit's fix is to only remove blocks when they haven't been marked reachable and all of its predecessors have been visited (this latter bit being the thing that irreducible control flow broke). To implement this, we pass in the already-computed ControlFlowGraph from the Context into the EgraphPass so that we can easily iterate of a block's predecessors.

Fixes #13365

[cfallin]

I guess I come back to the point I made here: we now have a pretty complex approach (with a very intricate proof, to your credit!) that only mostly satisfies the requirements, but (with this patch) will sometimes not fully DCE dead branches. On the other hand, if we had a separate DFS-over-blocks to find reachable blocks, we would (with a pretty simple implementation we could be pretty certain about) have correct reachable-code computation in all cases, without having to conservatively over-approximate and leave some dead code in place. Perhaps we should consider that approach again? Can we at least measure its compile-time impact?

[fitzgen]

Yeah, I thought about that too. I'll investigate a little.