[SCEV] Canonicalise round-up idiom when some bits known (#197126) Since #174380, instcombine can clear some set bits in the added constant in expressions like this, when A has some known-clear low order bits. (A + 15) & ~15 This transformation is valid, but can make it harder for later passes to recognise this idiom for rounding up to a power of 2. This is causing the ARM MVE tail predication pass to fail on loops with a trip count which is a multiple of a small power of 2. The fix is to reverse the transformation when building SCEV expressions, canonicalising to always use the largest valid value for the added constant. Alive proofs: https://alive2.llvm.org/ce/z/hhndoW https://alive2.llvm.org/ce/z/_JYVat
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