[PPC] Set SP after loading data from stack frame, if no red zone is present

Follow-up to r280705: Make sure that the SP is only restored after all data
is loaded from the stack frame, if there is no red zone.

This completes the fix for https://llvm.org/bugs/show_bug.cgi?id=26519.

Differential Revision: https://reviews.llvm.org/D24466

llvm-svn: 282174