AMDGPU: Fix redundant FP spilling/assert in some functions

If a function has stack objects, and a call, we require an FP. If we
did not initially have any stack objects, and only introduced them
during PrologEpilogInserter for CSR VGPR spills, SILowerSGPRSpills
would end up spilling the FP register as if it were a normal
register. This would result in an assert in a debug build, or
redundant handling of the FP register in a release build.

Try to predict that we will have an FP later, although this is ugly.