Shape inference: fix updating an inequalities-and-equations entry (don't self-eliminate)

[lukstafi]

E.g. instead of calling perhaps_eliminate_row_var create a perhaps_extend_row_var that makes sure to add the new inequality sides to the entry. (Same for dim vars).

This is a work-in-progress update rather than a "bug report", but there's been a bit of a hiatus recently -- so writing this instead of a commit which I should have...

[lukstafi]

I am going to throw out the Fourier-Motzkin inspired approach and process pairwise inequalities, with generating fresh variables if necessary.