fix: Replace unused ArrayGet/Set with constrain if possibly out of bounds

[asterite]

Description

Problem

For #5296

Summary

In DIE, when we find an ArrayGet/Set that we could remove, if that instruction might be an index out of bounds we still remove it but leave a constrain check too.

Additional Context

For now the code is a bit messy, I just wanted to get it working first.

Also, it's not working for slices yet because we can't insert out of bounds checks if we don't know the slice length. For that we'll need to add the checks like we do for ArrayGet for slice (this could be done in a separate PR, though).

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 circuit sizes

Generated at commit: 74f9fe0a977fbded8f5e61df6102bd83cc90a725, compared to commit: 0ebf1fee471641db0bffcc8307d20327613c78c1

🧾 Summary (10% most significant diffs)

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
hashmap	+760 ❌	+0.37%	+860 ❌	+0.23%

---

Full diff report 👇

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
hashmap	206,694 (+760)	+0.37%	367,891 (+860)	+0.23%

[jfecher]

Looks like a lot of special handling is required for inserting the asserts, particularly with arrays of struct elements. How does this code compare with if we always inserted the asserts, but added an extra constant folding pass after DIE to de-duplicate them?

[jfecher]

On topic: I would have expected us to be performing checks on the length of the array in the array type, not the size of the array in fields. E.g. using your example I would have expected code like:

After replacing unused array instructions with bounds checks:
acir(inline) fn main f0 {
  b0(v0: [Field, Field, Field; 3], v1: Field):
    v20 = cast v1 as u32
    v21 = mul v20, u32 3
    v22 = array_get v0, index v21
    v28 = lt v20, u32 3
    constrain v28 == u1 1 '"Index out of bounds"'
    v29 = lt v20, u32 3
    constrain v29 == u1 1 '"Index out of bounds"'
    return v22
}

Hence why I expected deduplication of v28 and v29

[vezenovm]

LGTM!