[AArch64] Clean up callee-save CFI emission. NFC.

Summary:
Avoid special case for FP, LR CFI emission and just allow general
AArch64FrameLowering::emitCalleeSavedFrameMoves() to handle them.  Also,
stop recalculating the stack offsets in emitCalleeSavedFrameMoves()
since we can just reuse the previously calculated offset stored in the
MachineFrameInfo.

Depends on D17000

Reviewers: t.p.northover, rengolin, mcrosier, jmolloy

Subscribers: aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D17004

llvm-svn: 261885