rBreak Critical Edges and other MIR work
This PR is built on top of #32080.
This adds the basic depth-first traversals for MIR, preorder, postorder and reverse postorder. The MIR blocks are now translated using reverse postorder. There is also a transform for breaking critical edges, which includes the edges from `invoke`d calls (`Drop` and `Call`), to account for the fact that we can't add code after an `invoke`. It also stops generating the intermediate block (since the transform essentially does it if necessary already).
The kinds of cases this deals with are difficult to produce, so the test is the one I managed to get. However, it seems to bootstrap with `-Z orbit`, which it didn't before my changes.
Also adds a new set of passes to run just before translation that
"prepare" the MIR for codegen. Removal of landing pads, region erasure
and break critical edges are run in this pass.
Also fixes some merge/rebase errors.
Some blocks won't be translated at all because they aren't reachable at
the LLVM level, these need to be dealt with because they lack a
terminator and therefore trigger an LLVM assertion.
Other blocks aren't reachable because of codegen-time optimistions, for
example not dropping types that don't need it, often resulting in blocks
with no predecessors. We'll clean those up as well.
This is a fairly standard transform that inserts blocks along critical
edges so code can be inserted along the edge without it affecting other
edges. The main difference is that it considers a Drop or Call
terminator that would require an `invoke` instruction in LLVM a critical
edge. This is because we can't actually insert code after an invoke, so
it ends up looking similar to a critical edge anyway.
The transform is run just before translation right now.
