[NFC] Introduce a type to model memory operation

Summary: This is a first step before changing the types to llvm::Align and introduce functions to ease client code.

Reviewers: courbet

Subscribers: arsenm, sdardis, nemanjai, jvesely, nhaehnle, hiraditya, kbarton, jrtc27, atanasyan, jsji, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73785