[mlir][llvm] Store memory op metadata using op attributes.

The revision introduces operation attributes to store tbaa metadata on
load and store operations rather than relying using dialect attributes.
At the same time, the change also ensures the provided getters and
setters instead are used instead of a string based lookup. The latter
is done for the tbaa, access groups, and alias scope attributes.

The goal of this change is to ensure the metadata attributes are only
placed on operations that have the corresponding operation attributes.
This is imported since only these operations later on translate these
attributes to LLVM IR. Dialect attributes placed on other operations
are lost during the translation.

Reviewed By: vzakhari, Dinistro

Differential Revision: https://reviews.llvm.org/D143654