[llvm] [IR] Don't skip `catchswitch` when calling `BB::getFirstNonPHIOrDbgOrAlloca()` (PR #136056)

[David Majnemer via llvm-commits]

majnemer wrote:

My thinking was that the behavior of the caller should be as-if it just passed in a basic block which just contained a terminator like br. There are no call-sites in such a basic block so I had it continue on rather than return.

https://github.com/llvm/llvm-project/pull/136056