Make model builder robust to multiple value-like terms in the same equivalence class...

Make model builder robust to multiple value-like terms in the same equivalence class (#8150)

This changes our policy on handling when two value-like terms are in the same equivalence class. We now give preference to the "base" model value, and either warn or throw a debug exception.

After f7675b2, we can have the situation where e.g. (seq.nth seq.empty 1) is in the same equivalence class as a constant, where (seq.nth seq.empty 1) is considered value-like, despite being unevaluatable. We now ensure that the constant is taken as the representative of the equivalence class, and not this term.

It also removes assignable terms from consideration when assigning values to equivalence classes. In particular, (seq.nth seq.empty 1) is skipped, so a warning is not even given when the above occurs.

Fixes #8148.