[AArch64] Simplify prolog/epilog callee save/restore. NFC.

Summary:
Factor out common code for callee-save register pair calculation.  This
is intended to simplify follow-on changes that reduce the number of
registers saved/restored.

Depends on D16732

Reviewers: mcrosier, jmolloy, t.p.northover

Subscribers: aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D16734

llvm-svn: 259384