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

llvm-project/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp:1539:10:
 error: unused variable 'SWaitInst' [-Werror,-Wunused-variable]
    auto SWaitInst =
         ^
1 error generated.