[llvm] [PlaceSafepoints] Report usage error instead of asserting on invalid gc.safepoint_poll (PR #205520)

[Gustas Rove via llvm-commits]

TheMixas wrote:

Thanks for the review. Addressed both points. I added the safepoint-poll-wrong-type.ll and safepoint-poll-empty.ll tests, and fixed the indentation. Confirmed on a non-assertions build that only the missing-function case actually crashes, so I kept the wrong-type and empty asserts and only converted the null check. The two new tests are REQUIRES: asserts since they exercise those asserts.

https://github.com/llvm/llvm-project/pull/205520