[llvm] CodeGen: replace assertions with explicit errors for landinpad lowering (PR #71287)

[via llvm-commits]

AlexDenisov wrote:

@arsenm thank you for the review and for the comments, highly appreciated!

> This seems like it should be in the domain of the verifier, rather than codegen fatal error

Agreed, I did start with the verifier check, but it turned out there are many compatibility tests which would be broken by such a check ([just one example](https://github.com/llvm/llvm-project/blob/main/llvm/test/Bitcode/compatibility-3.6.ll#L707)).

Would it make sense to upgrade all these tests to the right type them?
I'll double check the behaviour of old versions then, to make sure that the existing types there are indeed invalid.

https://github.com/llvm/llvm-project/pull/71287