feat(perf): Follow array sets backwards in array set from get optimization

[vezenovm]

Description

Problem*

Part of general effort reduce Brillig bytecode sizes

Summary*

Follow-up to #6207

From comments inside the PR:

/// If we have an array set whose value is from an array get on the same array at the same index,
/// we can simplify that array set to the array we were looking to perform an array set upon.
///
/// Simple case:
/// v3 = array_get v1, index v2
/// v5 = array_set v1, index v2, value v3
///
/// If we could not immediately simplify the array set from its value, we can try to follow
/// the array set backwards in the case we have constant indices:
///
/// v3 = array_get v1, index 1
/// v5 = array_set v1, index 2, value [Field 100, Field 101, Field 102]
/// v7 = array_set mut v5, index 1, value v3
///
/// We want to optimize `v7` to `v5`. We see that `v3` comes from an array get to `v1`. We follow `v5` backwards and see an array set
/// to `v1` and see that the previous array set occurs to a different constant index.
///
/// For each array set:
/// - If the index is non-constant we fail the optimization since any index may be changed.
/// - If the index is constant and is our target index, we conservatively fail the optimization.
/// - Otherwise, we check the array value of the `array_set``. We will refer to this array as array'.
///   In the case above, array' is `v1` from `v5 = array set ...`
///   - If the original `array_set` value comes from an `array_get`, check the array in that `array_get` against array'. 
///   - If the two values are equal we can simplify.
///     - Continuing the example above, as we have `v3 = array_get v1, index 1`, `v1` is
///       what we want to check against array'. We now know we can simplify `v7` to `v5` as it is unchanged.
///   - If they are not equal, recur marking the current `array_set` array as the new array id to use in the checks

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Changes to Brillig bytecode sizes

Generated at commit: 1132f9c0fcc700897a1508a3c780b34818885786, compared to commit: 24309200f600ad20a51d9f2c6c53849466fccda4

🧾 Summary (10% most significant diffs)

Program	Brillig opcodes (+/-)	%
fold_complex_outputs	-115 ✅	-17.67%

---

Full diff report 👇

Program	Brillig opcodes (+/-)	%
fold_complex_outputs	536 (-115)	-17.67%

[TomAFrench]

Note that run-external-checks only runs nargo check on aztec packages so it'll only find issues which appear in the frontend. It won't pick up any SSA errors so you'll need to check that manually (or we can compile the contracts)

[TomAFrench]

I think it would be good to have some more documentation within the function about our reasoning about what we're doing at particular steps.