[llvm] [WebAssembly] Fix br type checking (PR #108746)

[Heejin Ahn via llvm-commits]

aheejin wrote:

Come to think of it, I don't think this is the right way to fix this problem. Currently the type checker does not handle
1. Polymorphic types produced by `br`, `unreachable`, or `throw`.
2. Block return types. This is also related to #107524.

Closing this for now.

https://github.com/llvm/llvm-project/pull/108746