[AArch64] Fix bug in large stack spill slot handling (PR27717)

Summary:
Fix bug in MachO path where a frame index offset would not be reserved
for handling large frames when an extra non-used callee-save register
was saved.  In the case where the extra register is reserved or not a
GPR (e.g. %FP in the MachO case), this would lead to the register
scavenger later failing when called from PrologEpilogInserter.

Reviewers: t.p.northover

Subscribers: aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D20185

llvm-svn: 269697