feat(ssa): Multiple slice mergers

[vezenovm]

Description

Problem*

Resolves #2599

Summary*

This PR implements the following (this is also partially outlined in the issue discussion):

1. When we have a push_back, we need to check whether the length == capacity. If they are not equal we need to actually perform an array_set rather than making a new array. Simply adding an element to a slice which is the result of a slice merger would potentially result in the element being included after placeholder data in the slice's SSA value. When the slice is later accessed the placeholder data will potentially be accessed where the element that was pushed back is expected.
2. In order to conditionally array_set or make a new slice value we will need to expose the merging of values (the merge_values method in flattening) to the entire SSA crate rather than it being part of the flattening pass' context.
3. When we have a pop_back we no longer construct a new array by directly popping off of the the SSA array value that represents a slice. Instead, we simply array_get the flattened values of a slice and reduce the dynamically tracked length by one. For example in this program:

fn merge_slices_push_then_pop(x: Field, y: Field) -> [Field] {
    let mut slice = [0; 2];
    if x != y {
        slice = slice.push_back(y);
        slice = slice.push_back(x);
    } else {
        slice = slice.push_back(x);
    }
    slice = slice.push_back(30);
    if x == 20 {
        slice = slice.push_back(20);
    }
    let (slice, elem) = slice.pop_back();
    assert(elem == 30);
    let (slice, elem) = slice.pop_back();
    assert(elem == x);
    slice
}

When we get to the first pop_back we have a slice of (length 5, [0, 0, y, x, 30, 0]). If we just pop_back the slice value directly we will get 0. So instead we just do an array_get at len - 1 and leave the slice the exact same. The simplified result where we want to return (slice, elem) is just ((length 4, [0, 0, y, x, 30, 0]), 30). So the slice stays the same and we just reduce the length.

I posted under the issue description how I thought insert and remove would be the most challenging to implement, but it is in fact just push_back and pop_back as placeholder information is only every attached to the end of the slice. We can keep slices sound as long as we accurately track the slice length and its internal elements. This PR moves to further ignore any values that are part of a slice's SSA array value, but past the dynamically tracked slice length.

Documentation

• [] This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • [] I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

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

[jfecher]

I haven't finished going through this PR, but can you fix the conflicts?

[vezenovm]

I haven't finished going through this PR, but can you fix the conflicts?

Yup will do, these look to be new ones that came up after the PR was made.