[IR] Refactor memtransfer inst classes (NFC)

Summary:
A simple refactor to remove duplicate code in the definitions of
MemTransferInst, AtomicMemTransferInst, and AnyMemTransferInst.
Introduce a templated base class that contains all of the methods
unique to a memory transfer intrinsic, and derive these three
classes from that.

llvm-svn: 329744