[ConstraintElim] Strengthen flags at the closest dominator of all uses. (#220918) Flags are currently only strengthened using the facts that hold on entry to the block defining the instruction. This overly pessimistic. It is sufficient to check at the closest instruction dominating all uses. Extend FactOrCheck to carry this point as context instruction (in union with the mutually exclusive DoesHold to avoid size growth) and retrieve the common dominating instruction when queuing instructions for flag strengthening. This strengthens flag inference in a number of real-world cases, as well as enables additional folds due stronger flags: https://github.com/dtcxzyw/llvm-opt-benchmark-nightly/pull/1171 Compile-time impact is in the noise https://llvm-compile-time-tracker.com/compare.php?from=11bc6ce46e227718c3b54aa6889d7458656e8ab4&to=6f67fdcdb62a5fe56060cb266448db8b763e13d7&stat=instructions:u PR: https://github.com/llvm/llvm-project/pull/220918
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