[ConstraintElim] Replace custom GEP index handling by using existing code

[ConstraintElim] Replace custom GEP index handling by using existing code

Instead of duplicating the existing decomposition code for GEP indices
just use the existing code by calling the existing decompose function on
the index expression and multiply the result's coefficients by the scale of
the index.

This both reduces code duplication and generalizes the pattern we can
handle.