[AMDGPU] We would need FP if there is call and caller save VGPR spills

Since https://reviews.llvm.org/D98319, determineCalleeSavesSGPR() needs
to consider caller save VGPR spills as well while anticipating if we
require FP.

Fixes: SWDEV-295978

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D106758