[llvm] [CodeGen] Diagnose unsupported landingpad result types instead of asserting (PR #223205)

[Oliver Hunt via llvm-commits]

ojhunt wrote:

This feels like it isn't correct - specifically the use of `ValueVTs.size() != 2` feels like a copy of just the assertion condition (e.g. suppressing the assertion by construction rather than correct state).

It seems like this is probably best done by examining landing pad element types, not just the count, and that should probably happen earlier

https://github.com/llvm/llvm-project/pull/223205