[llvm] Fix -Wunused-variable in ConstantFold.cpp (NFC)

/llvm-project/llvm/lib/IR/ConstantFold.cpp:137:12: error: unused variable 'CSize' [-Werror,-Wunused-variable]
  unsigned CSize = cast<IntegerType>(C->getType())->getBitWidth()/8;
           ^
1 error generated.