[mlir] [VectorOps] Improve scatter/gather CPU performance

[mlir] [VectorOps] Improve scatter/gather CPU performance

Replaced the linearized address with the proper LLVM way of
defining vector of base + indices in SIMD style. This yields
much better code. Some prototype results with microbencmarking
sparse matrix x vector with 50% sparsity (about 2-3x faster):

         LINEARIZED     IMPROVED
GFLOPS  sdot  saxpy     sdot saxpy
16x16    1.6   1.4       4.4  2.1
32x32    1.7   1.6       5.8  5.9
64x64    1.7   1.7       6.4  6.4
128x128  1.7   1.7       5.9  5.9
256x256  1.6   1.6       6.1  6.0
512x512  1.4   1.4       4.9  4.7

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D84368