[mlir][VectorOps] Implement strided_slice conversion

Summary:
This diff implements the progressive lowering of strided_slice to either:
  1. extractelement + insertelement for the 1-D case
  2. extract + optional strided_slice + insert for the n-D case.

This combines properly with the other conversion patterns to lower all the way to LLVM.

Appropriate tests are added.

Reviewers: ftynse, rriddle, AlexEichenberger, andydavis1, tetuante

Reviewed By: andydavis1

Subscribers: merge_guards_bot, mehdi_amini, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72310