[GlobalISel] Remove unused variable 'Or' (NFC)

/Users/jiefu/llvm-project/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp:3450:10: error: unused variable 'Or' [-Werror,-Wunused-variable]
    auto Or = MIRBuilder.buildOr(CarryOut, And, Res_ULT_LHS);
         ^
1 error generated.