[Plonky3 Codegen] Investigate impact of aux trace generation on CSE pass

In #515, we introduce aux trace generation helpers for the Plonky3 backend.
This adds nodes to the algebraic graph to represent the numerators and denominators of buses transition constraints.

However, adding nodes to the algebraic graph may prevent the CSE pass to do some optimization (as all the nodes should still be in the graph).

We should investigate the impact and see if we need an alternative solution (for instance, keeping track of the numerator / denominator outside the algebraic graph).

_Originally posted by @Al-Kindi-0 in https://github.com/0xMiden/air-script/pull/515#discussion_r2588906655_

Examine the Plonky3 backend codegen, particularly the algebraic graph and CSE (common subexpression elimination) pass introduced in PR #515. Look at how aux trace generation helpers add nodes for numerators and denominators of buses transition constraints. Assess whether these nodes hinder CSE optimizations and consider alternative designs, such as tracking numerators/denominators outside the graph. Start by reviewing the relevant files in the codegen module and the CSE pass logic.

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