A separate AVX512 lowering pass does not compose well with the regular
vector lowering pass. As such, it is at risk of code duplication and
lowering inconsistencies. This change removes the separate AVX512 lowering
pass and makes it an "option" in the regular vector lowering pass
(viz. vector dialect "augmented" with AVX512 dialect).
Reviewed By: rriddle
Differential Revision: https://reviews.llvm.org/D92614
