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At the moment if we decide to enable tail-folding we do not include the cost of generating the mask per VF. This can mean we make some poor choices of VF, which is definitely true for SVE-enabled AArch64 targets where mask generation for fixed-width vectors is more expensive than for scalable vectors. I've added a VPInstruction::computeCost function to return the costs of the ActiveLaneMask and ExplicitVectorLength operations. Unfortunately, in order to prevent asserts firing I've also had to duplicate the same code in the legacy cost model to make sure the chosen VFs match up. I've wrapped this up in a ifndef NDEBUG for now. The alternative would be to disable the assert completely when tail-folding, which I imagine is just as bad. New tests added: Transforms/LoopVectorize/AArch64/sve-tail-folding-cost.ll Transforms/LoopVectorize/RISCV/tail-folding-cost.ll
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