Substitution in order should take care of NegativeInfinity

Fixes #7872 

The following substiution now works:
```
In [1]: O(x**3).subs(x, exp(-x**2))
Out[1]: 
 ⎛     2        ⎞
 ⎜ -3⋅x         ⎟
O⎝ℯ     ; x → -∞⎠
```