[AMDGPU] Free reserved VGPR if no SGPR spill

I met some code generation behavior change when I tried to remove
the hasStackObject() check when reserving VGPR for SGPR spill.
For example, the function `callee_no_stack_no_fp_elim_all` in the lit
test file `callee-frame-setup.ll`.
The generated code changed from:
```
  s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
  s_mov_b32 s4, s33
  s_mov_b32 s33, s32
  s_mov_b32 s33, s4
  s_setpc_b64 s[30:31]
```

into something like:
```
  s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
  v_writelane_b32 v63, s33, 0
  s_mov_b32 s33, s32
  v_readlane_b32 s33, v63, 0
  s_setpc_b64 s[30:31]
```

I think we still prefer the old version where only scalar instructions are needed.
The idea here is free the reserved VGPR if no SGPR spills. So we will very likely
to use a free SGPR for fp/sp spill.

Reviewed by: arsenm

Differential Revision: https://reviews.llvm.org/D98344