[AArch64] NFC: Refactoring to prepare for vector PCS.

This patch refactors several parts of AArch64FrameLowering
so that it can be easily extended to support saving/restoring
of FPR128 (Q) registers.

Reviewers: t.p.northover, gberry, thegameg, rengolin, javed.absar

Reviewed By: thegameg

Differential Revision: https://reviews.llvm.org/D51478

llvm-svn: 342038