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.
