fix(acir_gen): A slice might be a nested Array, not a flattened DynamicArray

[aakoshh]

Description

Problem*

Resolves #9593

Summary*

Fixes convert_ssa_intrinsic_call handling SlicePopFront where it calculated the number of items it needs to pop wrong: it assumed the slice will be a flattened DynamicArray, but in this case it was an Array with a nested structure. Based on this assumption it popped as many elements as were in the item, rather than just 1.

Additional Context

This is what the slice looked like as an AcirValue:

            Array([
                Array([
                    Var(AcirVar(30), NumericType(Unsigned { bit_size: 8 })), 
                    Var(AcirVar(38), NumericType(Unsigned { bit_size: 8 }))
                ]), 
                Array([
                    Var(AcirVar(47), NumericType(Unsigned { bit_size: 8 })), 
                    Var(AcirVar(55), NumericType(Unsigned { bit_size: 8 }))
                ]), 
                Array([
                    Var(AcirVar(59), NumericType(Unsigned { bit_size: 8 })), 
                    Var(AcirVar(62), NumericType(Unsigned { bit_size: 8 }))
                ])
            ])

The routine calculated that each item consists of 2 characters, and therefore it popped the first 2 arrays, leaving new_slice to be just a single array:

[
    Array([
        Var(AcirVar(59), NumericType(Unsigned { bit_size: 8 })), 
        Var(AcirVar(62), NumericType(Unsigned { bit_size: 8 }))
    ])]

I have not tried if any of the other operations have a similar assumption. I think I'll add Insert and Remove to the fuzzer to cover them, as they have quite substantial ACIR gen logic.

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]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Execution Time'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: aed4ebd	Previous: ef436d0	Ratio
private-kernel-inner	0.018 s	0.014 s	1.29

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Test Suite Duration'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: aed4ebd	Previous: ef436d0	Ratio
test_report_zkpassport_noir-ecdsa_	3 s	2 s	1.50
test_report_zkpassport_noir_rsa_	2 s	1 s	2

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[vezenovm]

Looks good! Would be good to get similar tests for insert and remove as well as you mentioned in the OP.