[mlir][IRNumbering] Fix the dialect comparator to be strict

[mlir][IRNumbering] Fix the dialect comparator to be strict

Check if rhs is the dialect to be ordered first, ensuring that
we don't inadvertantly order something before it by
falling back to pure number comparison.

This only shows up depending on the implementation of
stable_sort. This was hit in a build of MSVC that was
checking for strict ordering.