[mlir][llvm] Improve LoadOp and StoreOp import.

The revision supports importing the volatile keyword and nontemporal
metadata for the LoadOp and StoreOp. Additionally, it updates the
builders and uses an assembly format for printing and parsing.

The operation type still requires custom parse and print methods
due to the current handling of typed and opaque pointers.

Reviewed By: Dinistro

Differential Revision: https://reviews.llvm.org/D143714