[mlir] [VectorOps] Enable 32-bit index optimizations

Rationale:
After some discussion we decided that it is safe to assume 32-bit
indices for all subscripting in the vector dialect (it is unlikely
the dialect will be used; or even work; for such long vectors).
So rather than detecting specific situations that can exploit
32-bit indices with higher parallel SIMD, we just optimize it
by default, and let users that don't want it opt-out.

Reviewed By: nicolasvasilache, bkramer

Differential Revision: https://reviews.llvm.org/D87404