[VPlan] Perform optimizeMaskToEVL in terms of pattern matching (#155394) Currently in optimizeMaskToEVL we convert every widened load, store or reduction to a VP predicated recipe with EVL, regardless of whether or not it uses the header mask. So currently we have to be careful when working on other parts VPlan to make sure that the EVL transform doesn't break or transform something incorrectly, because it's not a semantics preserving transform. Forgetting to do so has caused miscompiles before, like the case that was fixed in #113667 This PR rewrites it to work in terms of pattern matching, so it now only converts a recipe to a VP predicated recipe if it is exactly masked with the header mask. After this the transform should be a true optimisation and not change any semantics, so it shouldn't miscompile things if other parts of VPlan change. This fixes #152541, and allows us to move addExplicitVectorLength into tryToBuildVPlanWithVPRecipes in #153144 It also splits out the load/store transforms into separate patterns for reversed and non-reversed, which should make #146525 easier to implement and reason about.
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