[AMDGPU] Fix -Wunused-variable in SIInsertWaitcnts.cpp (NFC)

llvm-project/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp:1322:10:
 error: unused variable 'SWaitInst' [-Werror,-Wunused-variable]
    auto SWaitInst =
         ^
llvm-project/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp:1334:10:
 error: unused variable 'SWaitInst' [-Werror,-Wunused-variable]
    auto SWaitInst = BuildMI(Block, It, DL, TII->get(AMDGPU::S_WAITCNT_VSCNT))
         ^
2 errors generated.