[AArch64][Falkor] Avoid HW prefetcher tag collisions (step 1)

Summary:
This patch is the first step in reducing HW prefetcher instruction tag
collisions in inner loops for Falkor.  It adds a pass that annotates IR
loads with metadata to indicate that they are known to be strided loads,
and adds a target lowering hook that translates this metadata to a
target-specific MachineMemOperand flag.

A follow on change will use this MachineMemOperand flag to re-write
instructions to reduce tag collisions.

Reviewers: mcrosier, t.p.northover

Subscribers: aemerson, rengolin, mgorny, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D34963

llvm-svn: 308059