[IR] Refactor memtransfer inst classes (NFC)

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329744 91177308-0d34-0410-b5e6-96231b3b80d8