[mlir][vector] add higher dimensional support to gather/scatter

[mlir][vector] add higher dimensional support to gather/scatter

Similar to mask-load/store and compress/expand, the gather and
scatter operation now allow for higher dimension uses. Note that
to support the mixed-type index, the new syntax is:
   vector.gather %base [%i,%j] [%kvector] ....
The first client of this generalization is the sparse compiler,
which needs to define scatter and gathers on dense operands
of higher dimensions too.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D97422