UseListOrder: Fix undefined behaviour

This commit fixes undefined behaviour that caused the revert in r214249.

The problem was two unsequenced operations on a `DenseMap<>`, giving
different behaviour in GCC and Clang.  This:

    DenseMap<T*, unsigned> DM;
    for (auto &X : ...)
      DM[&X] = DM.size() + 1;

should have been:

    DenseMap<T*, unsigned> DM;
    for (auto &X : ...) {
      unsigned Size = DM.size();
      DM[&X] = Size + 1;
    }

Until r214242, this difference between compilers didn't matter.  In
r214242, `OrderMap::LastGlobalValueID` was introduced and compared
against IDs, which in GCC were off-by-one my expectations.