[llvm] [LLVM] Fix assertion when dropping type tests with SelectInst users (PR #185523)

[Paul Kirth via llvm-commits]

https://github.com/ilovepi approved this pull request.

LGTM modulo some tiny nits. Give @pcc a chance to chime in though incase I'm failing to see some interaction here w/ the downstream consumers of the type test, like CFI and devirtualization.

https://github.com/llvm/llvm-project/pull/185523