[AMDGPU] Remove -amdgpu-spill-sgpr-to-smem.

Summary: The implementation was never completed and never used except in tests.

Reviewers: arsenm, mareko

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69163

llvm-svn: 375293