[AArch64] Enable clustering memory accesses to fixed stack objects

Summary:
r347747 added support for clustering mem ops with FI base operands
including support for fixed stack objects in shouldClusterFI, but
apparently this was never tested.

This patch fixes shouldClusterFI to work with scaled as well as
unscaled load/store instructions, and fixes the ordering of memory ops
in MemOpInfo::operator< to ensure that memory addresses always
increase, regardless of which direction the stack grows.

Subscribers: MatzeB, kristof.beyls, hiraditya, javed.absar, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71334