[AMDGPU] Remove unused variable AllSGPRSpilledToVGPRs. NFC

Summary:
Removing the unused variable AllSGPRSpilledToVGPRs in
SIFrameLowering::processFunctionBeforeFrameFinalized
to avoid
  error: variable 'AllSGPRSpilledToVGPRs' set but not used
  [-Werror=unused-but-set-variable]

Reviewers: arsenm, nhaehnle

Reviewed By: nhaehnle

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63721

llvm-svn: 364190