[AMDGPU] Fix warnings about unused variables [NFC]

Without this gcc warned like
 ../lib/Target/AMDGPU/GCNDPPCombine.cpp:371: warning: unused variable 'OpSelOpr' [-Wunused-variable]
   371 |       if (auto *OpSelOpr =
       |
 ../lib/Target/AMDGPU/GCNDPPCombine.cpp:388: warning: unused variable 'OpSelHiOpr' [-Wunused-variable]
   388 |       if (auto *OpSelHiOpr =
       |

Remove the unused variables.