[TargetLowering] Change getOptimalMemOpType to take a function attribute list

The MachineFunction wasn't used in getOptimalMemOpType, but more importantly,
this allows reuse of findOptimalMemOpLowering that is calling getOptimalMemOpType.

This is the groundwork for the changes in D59766 and D59787, that allows
implementation of TTI::getMemcpyCost.

Differential Revision: https://reviews.llvm.org/D59785

llvm-svn: 359537