[AMDGPU] Do not reserve any VGPR for SGPR spills

After the split register allocation changes in eebe841a it is no
longer necessary to reserve a VGPR before RA. This can also create bugs
when IPRA is enabled since we cannot predict that a called function may
not reserve any register if it does not have any SGPR spills. If that
happens those functions may override reserved registers that are
normally callee saved. Added a test to show this.

Fixes: SWDEV-309900

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D115551