[CodeGen] Fix -Wunused-variable in RegisterCoalescer.cpp (NFC)

/llvm-project/llvm/lib/CodeGen/RegisterCoalescer.cpp:1320:18: error: unused variable 'DefSubIdx' [-Werror,-Wunused-variable]
  const unsigned DefSubIdx = DefMI->getOperand(0).getSubReg();
                 ^
1 error generated.