[AMDGPU] Remove assertion on S1024 SGPR to VGPR spill

Summary:
Replace an assertion that blocks S1024 SGPR to VGPR spill.
The assertion pre-dates S1024 and is not wave size dependent.

Reviewers: arsenm, sameerds, rampitec

Reviewed By: arsenm

Subscribers: qcolombet, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80783