[mlir] [VectorOps] Improve SIMD compares with narrower indices

When allowed, use 32-bit indices rather than 64-bit indices in the
SIMD computation of masks. This runs up to 2x and 4x faster on
a number of AVX2 and AVX512 microbenchmarks.

Reviewed By: bkramer

Differential Revision: https://reviews.llvm.org/D87116