Updating xarch to utilize EVEX compares and blending where possible

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-- This is an early draft and still requires more work. In particular there still needs to be a step, likely in lowering, which recognizes cases of CvtMaskToVector(Compare(x, y)) and uses the legacy SIMD comparison instead. The same goes for cases like BlendVariable(x, y, CvtVectorToMask(z))

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