AMDGPU/NFC: Rename few things in SIMemoryLegalizer:

  - AtomicInfo -> MemOpInfo
  - getAtomicLoadInfo -> getLoadInfo
  - getAtomicStoreInfo -> getStoreInfo
  - expandAtomicLoad -> expandLoad
  - expandAtomicStore -> expandStore

Differential Revision: https://reviews.llvm.org/D36861

llvm-svn: 311179