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


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@342038 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm