[VPlan] Fix header masks in EVL tail folding (#150202) With EVL tail folding, the EVL may not always be VF on the second-to-last iteration. Recipes that have been converted to VP intrinsics via optimizeMaskToEVL account for this, but recipes that are left behind will still use the old header mask which may end up having a different vector length. This is effectively the same as #95368, and fixes this by converting header masks from icmp ule wide-canonical-iv, backedge-trip-count -> icmp ult step-vector, evl. Without it, recipes that fall through optimizeMaskToEVL may use the wrong vector length, e.g. in #150074 and #149981. We really need to split off optimizeMaskToEVL into VPlanTransforms::optimize and move transformRecipestoEVLRecipes into tryToBuildVPlanWithVPRecipes, so we don't mix up what is needed for correctness and what is needed to optimize away the mask computations. We should be able to still generate a correct albeit suboptimal VPlan without running optimizeMaskToEVL. I've added a TODO for this, which I think we can do after #148274 Fixes #150197
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