[mlir][llvm] Add atomic support to the LoadOp.

This revision adds atomic support to the LoadOp. It chooses
to print the atomic keywords together with the syncscope and
ordering arguments, which simplifies parsing and printing compared
to the LLVM IR printer that puts the atomic keyword at the beginning.
It uses the ordering attribute to check if the load is atomic.

The revision also implements verifiers to ensure the constraints
that apply to atomic load operations are checked.

Reviewed By: Dinistro

Differential Revision: https://reviews.llvm.org/D144112