This lowers exception catching-related instructions:
1. Lowers `wasm.catch` intrinsic to `catch` instruction
2. Removes `catchpad` and `cleanuppad` instructions; they are not necessary after isel phase (`MachineBasicBlock::isEHFuncletEntry()` or `MachineBasicBlock::isEHPad()` can be used instead.)
3. Lowers `catchret` and `cleanupret` instructions to pseudo `catchret` and `cleanupret` instructions in isel, which will be replaced with other instructions in `WebAssemblyExceptionPrepare` pass.
4. Adds 'WebAssemblyExceptionPrepare` pass, which is for running various transformation for EH.ReplaceFuncletReturns` pass, Currently this pass only replaces `catchret` and `cleanupret` instructions into appropriate wasm instructions to make this patch successfully run until the endwhich replaces `catchret`/`cleanupret` instructions with `br` and `rethrow` appropriately.
Currently this does not handle lowering of intrinsics related to LSDA info generation (`wasm.landingpad.index` and `wasm.lsda`), because they cannot be tested without implementing `EHStreamer`'s wasm-specific handlers. They are marked as TODO, which is needed to make isel pass. Also this does not generate `try` and `end_try` markers yet, which will be handled in later patches.
This patch is based on [[ https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md | the first wasm EH proposal ]].