[AARCH64] Teach AArch64FrameLowering::getFrameIndexReferencePreferSP really prefer SP.

Do more efforts to use sp if it is possible to lower a frame index.

Reviewers: reames, loicottet, ostannard, t.p.northover
Reviewed By: reames
Subscribers: arphaman, danilaml, hiraditya, kristof.beyls, llvm-commits, Matt, yrouban
Differential Revision: https://reviews.llvm.org/D111133