Slice operations do not handle correctly slices of structs

[sirasistant]

Aim

Slices of structs should generate consistent SSA

Expected Behavior

Consider the following code:

struct NestedSliceStruct {
    id: Field,
    arr: [Field]
}

unconstrained fn create_struct(id: Field, value: Field) -> NestedSliceStruct {
    let mut arr = [id];
    arr = arr.push_back(value);
    NestedSliceStruct { id, arr }
}

unconstrained fn main(a: Field, b: Field) {
    let mut slice = [create_struct(a, b), create_struct(b, a)];

    let item_that_will_be_removed = slice[1];
    assert(item_that_will_be_removed.id == b);

    let remove_result = slice.remove(1);
    let removed_item = remove_result.1;
...
}

It should generate SSA like the following:

brillig fn main f0 {
  b0(v0: Field, v1: Field):
    v3, v4 = call f1(v0, v1)
    v6, v7 = call f1(v1, v0)
    v9 = allocate
    store [v3, v4, v6, v7] at v9
    v10 = load v9
    v13 = array_get v10, index Field 2
    v15 = array_get v10, index Field 3
    v16 = eq v13, v1
    constrain v16
    v18 = load v9
    v19, v20, v21 = call slice_remove(v18, Field 2)
...
}

Bug

The ssa generated looks like this:

brillig fn main f0 {
  b0(v0: Field, v1: Field):
    v3, v4 = call f1(v0, v1)
    v6, v7 = call f1(v1, v0)
    v9 = allocate
    store [v3, v4, v6, v7] at v9
    v10 = load v9
    v13 = array_get v10, index Field 2
    v15 = array_get v10, index Field 3
    v16 = eq v13, v1
    constrain v16
    v18 = load v9
    v19, v20, v21 = call slice_remove(v18, Field 1) // here: The index should have been multipled by the number of elements in the struct  
    // Like it has been for array_get operations
    v22 = eq v20, v1
    constrain v22
    v24 = array_get v21, index Field 0
    v25 = eq v24, Field 0
    constrain v25
    return 
}

Also, the simplification in SSA does not handle correctly struct slices:
https://github.com/noir-lang/noir/blob/master/crates/noirc_evaluator/src/ssa_refactor/ir/instruction.rs#L438
As you can see there, it's assuming there is only going to be one parameter to push to a slice. But when pushing to struct slices, the SSA does have one parameter per field of the struct:

brillig fn push_back_to_slice f2 {
  b0(v0: [Field, [Field]], v1: Field, v2: [Field]):
    v4 = call slice_push_back(v0, v1, v2)
    return v4
}

To Reproduce

Installation Method

None

Nargo Version

No response

Additional Context

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Would you like to submit a PR for this Issue?

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