[AArch64] Simplify callee-save register save/restore. NFC.

Summary:
Simplify callee-save register save/restore code generation by
remembering the size of the callee-save area when it is computed so we
don't have to scan the prologue/epilogue instructions again later to
reconstruct it.

This is intended to simplify follow-on changes that reduce the number of
registers saved/restored.

Reviewers: mcrosier, jmolloy, t.p.northover

Subscribers: aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D16732

llvm-svn: 259365