Not all PPC functions with a frame pointer need a RS spill slot

We used to add a spill slot for the register scavenger whenever the function
has a frame pointer. This is unnecessarily conservative: We may need the spill
slot for dynamic stack allocations, and functions with dynamic stack
allocations always have a FP, but we might also have a FP for other reasons
(such as the user explicitly disabling frame-pointer elimination), and we don't
necessarily need a spill slot for those functions.

The structsinregs test needed adjustment because it disables FP elimination.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177106 91177308-0d34-0410-b5e6-96231b3b80d8