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

[Gustas Rove via llvm-commits]

TheMixas wrote:

@pektezol @dtcxzyw I opened this as my first contribution. It converts the three gc.safepoint_poll precondition asserts in InsertSafepointPoll to reportFatalUsageError, with a regression test for the missing-function case. Would appreciate a review when you have a moment.

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