[mlir][sparse] add option for 32-bit indices in scatter/gather

[mlir][sparse] add option for 32-bit indices in scatter/gather

Controlled by a compiler option, if 32-bit indices can be handled
with zero/sign-extention alike (viz. no worries on non-negative
indices), scatter/gather operations can use the more efficient
32-bit SIMD version.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D103632