[LoopRotate] Use SCEV exit counts to improve rotation profitability (#187483) Most loop transformations, like unrolling and vectorization, expect the latch branch to be countable. Allow rotation, if it turns the latch from uncountable to countable. This use SCEV to check for countable exits, if CheckExitCount set. Currently it is not set for the LPM1 run (where SCEV is not used by other passes), only in LPM. With that compile-time impact is mostly neutral https://llvm-compile-time-tracker.com/compare.php?from=eba342d0ba930a404a026c80aada51c43974f0db&to=2e676337b45fae63ce9498116d8e6e43772363c5&stat=instructions:u ClamAV is consistently slower (~+0.15%) and 7zip faster in most cases (~-0.13%) Across a large test set based on C/C++ workloads, this rotates ~0.8% more loops with ~2.68M rotated loops. For the test set, ~2.7% more loops are runtime-unrolled and +6.36% more early exit loops vectorized on ARM64 macOS. This fixes a regression where std::ranges::find_last loops stopped being runtime-unrolled after https://github.com/llvm/llvm-project/commit/5f648c370edf5d71c471ffbabdaaa821ad05fb4b which changed the loop structure so we stopped rotating. https://clang.godbolt.org/z/6baeE1av6 Based on https://github.com/llvm/llvm-project/pull/162654. Co-authored-by: Marek Sedláček <mr.mareksedlacek@gmail.com> PR: https://github.com/llvm/llvm-project/pull/187483
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