Suppress output even when a comment follows ;. Fixes #4525.

Fixes https://github.com/ipython/ipython/issues/4525 and https://github.com/JuliaLang/IJulia.jl/issues/109

Before:

    In [1]: 1
    Out[1]: 1
    
    In [2]: 1;

    In [3]: 1; # comment
    Out[3]: 1

    In [4]: 1; 2
    Out[4]: 2

    In [5]: 1 # comment ;

Now:

    In [1]: 1
    Out[1]: 1

    In [2]: 1;

    In [3]: 1; # comment

    In [4]: 1; 2
    Out[4]: 2

    In [5]: 1 # comment;
    Out[5]: 1