[AMDGPU] Spill CSR VGPR which is reserved for SGPR spills

Update logic for reserving VGPR for SGPR spills. A CSR VGPR being reserved for
SGPR spills could be clobbered if there were no free lower VGPR's available.
Create a stack object so that it will be spilled in the prologue. Also
adds more tests.

Differential Revision: https://reviews.llvm.org/D83730