[mlir][Affine] Introduce affine.vector_load and affine.vector_store

This patch adds `affine.vector_load` and `affine.vector_store` ops to
the Affine dialect and lowers them to `vector.transfer_read` and
`vector.transfer_write`, respectively, in the Vector dialect.

Reviewed By: bondhugula, nicolasvasilache

Differential Revision: https://reviews.llvm.org/D79658