[mlir] add support for index type in vectors.

The patch enables the use of index type in vectors. It is a prerequisite to support vectorization for indexed Linalg operations. This refactoring became possible due to the newly introduced data layout infrastructure. The data layout of a module defines the bitwidth of the index type needed to verify bitcasts and similar vector operations.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D99948