[mlir] Add unsigned version of index_cast

This is required to be able to cast integer type to a potential larger index using zero-extend cast.

There is a larger change under discussion to move index ops in a separate dialect: https://discourse.llvm.org/t/rfc-index-dialect/65540/
Based on timing of this work this patch can be included as part of this effort but as a short term solution we may want to add this op to arithmetic dialect for now in order to fill the gap.

Reviewed By: Mogball, stellaraccident

Differential Revision: https://reviews.llvm.org/D135089