[NFC] Encapsulate MemOp logic

Summary:
This patch simply introduces functions instead of directly accessing the fields.
This helps introducing additional check logic. A second patch will add simplifying functions.

Reviewers: courbet

Subscribers: arsenm, nemanjai, jvesely, nhaehnle, hiraditya, kbarton, jsji, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73945