[ARM] Treat memcpy/memset/memmove as call instructions for low overhead loops

[ARM] Treat memcpy/memset/memmove as call instructions for low overhead loops

If an instruction will be lowered to a call there is no advantage of
using a low overhead loop as the LR register will need to be spilled and
reloaded around the call, and the low overhead will end up being
reverted. This teaches our hardware loop lowering that these memory
intrinsics will be calls under certain situations.

Differential Revision: https://reviews.llvm.org/D90439