[NFC] Introduce an API for MemOp

Summary: This patch introduces an API for MemOp in order to simplify and tighten the client code.

Reviewers: courbet

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73964