[AMDGPU] Avoid assert for saved FP

With spilling into AGPRs enabled we cannot reliably predict
if we need to save FP or not. We may finally spill everything
into AGPRs and never touch stack. In this case we still may
save FP. This is deficiency but not an error, so avoid the
assert.

Differential Revision: https://reviews.llvm.org/D107404