[llvm] [LoopVectorize] Don't speculate an early-exit trip count that may cause UB (PR #219893)

[Madhur Amilkanthwar via llvm-commits]

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madhur13490 wrote:

Added udiv_by_poison_step_early_exit_load (early exit on a loaded byte being zero) plus a non-poison counterpart that still vectorizes, confirming the poison case is rejected only due to the UB trip count.

https://github.com/llvm/llvm-project/pull/219893