[llvm] [SimplifyCFG] Don't delete basic block if it is a partial cleanuppad (PR #157363)

[Gábor Spaits via llvm-commits]

spaits wrote:

I have checked the failing tests in the CI and I have found something pretty interesting.

It is not caused by the PR, it is rather "found" by it. I have already worked around it.
In the following tests:
```
CodeGen/WebAssembly/cfg-stackify-eh-legacy.ll
CodeGen/WebAssembly/cfg-stackify-eh.ll
Transforms/Coroutines/coro-catchswitch-cleanuppad.ll
```
it may happen, that the `catchpad`'s `within` argument is a poison due to a deleted `catchswitch` block. According to the language reference it shouldn't be that (https://llvm.org/docs/LangRef.html#id355).

It doesn't seem cause any problems for now.

https://github.com/llvm/llvm-project/pull/157363