[WebAssembly] Remove WasmEHFuncInfo (NFC) (#194972)
This removes `WasmEHFuncInfo` class.
This class was created to maintain the information of, "If an exception
is not caught by EHPad A, what is its next unwind destination?". Turns
out this information is already in the CFG.
After #130374, we use the common `findUnwindDestination`:
https://github.com/llvm/llvm-project/blob/113479d119a997e4c4c3eae63e087588c9662121/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp#L2107-L2164
Note that in case of `catchswitch`, we follow its unwind destination
chain and add all of them to the invoke BB's successors until it meets a
`cleanuppad`, which always catches an exception. And the order of the
successor is the order of the unwind destination chain. So an invoke
BB's successor list would be like: [normal destination, unwind EHPad 1,
unwind EHPad 2, unwind EHPad 3, ...] where EHPad 2 is the next unwind
destination if EHPad 1 does not catch an exception and so on. So if we
want to know what the current EHPad's next unwind destination is, we can
examine the EHPad's predecessor (invoke BB)'s successor list, and find
the next successor of EHPad. If there is no further next unwind
destination, the list should end with that EHPad.
How we do this is in the added lambda functions in CFGStackify, which
replaces `WasmEHFuncInfo`'s `hasUnwindDest`/`getUnwindDest`.
This depends on the order of successors of a CFG, which someone can
argue may be not guaranteed, in case someone messes up the successor
order in later passes. But I think in practice this should be fine. Also
maintaining `WasmEHFuncInfo` has more risks, because if other passes
change EH-related CFGs, they have to make sure `WasmEHFuncInfo` stays
synced. I haven't personally found (so far) those passes and haven't had
to any adjustments to other passes, but it was always an inherent risk
when you maintain a separate data structure.
I wasn't able to completely delete `WasmEHFuncInfo.h` because it
contains the `Tag` enum. I renamed it to `WasmEHInfo.h`, because we
dont' have `WasmEHFuncInfo` class anymore.
