This adds assembly parsing support for the new EH (exnref) proposal.
`try_table` parsing is a little tricky because catch clause lists use
`()` and the multivalue block return types also use `()`. This handles
all combinations below:
- No return type (void) + no catch list
- No return type (void) + catch list
- Single return type + no catch list
- Single return type + catch list
- Multivalue return type + no catch list
- Multivalue return type + catch list
This does not include AsmTypeCheck support yet. That's the reason why
this adds a new test file and use `--no-type-check` in the command line.
After the type checker is added as a follow-up, I plan to merge
https://github.com/llvm/llvm-project/blob/main/llvm/test/MC/WebAssembly/eh-assembly-legacy.s
with this file. (Turning on `-mattr=+exception-handling` adds support
for all legacy and new EH instructions in the assembly.
`-wasm-enable-exnref` in `llc` only controls which instructions to
generate and it doesn't affect `llvm-mc` and assembly parsing.)
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