feat(ssa): Merge slices in if statements with witness conditions

[vezenovm]

Description

Problem*

Resolves #1889

Summary*

This PR enables merging of slice values in if statements where the condition is a witness. This change was preceded by #2243 as merging of slices occurs in the flattening pass and requires simplification of the slice intrinsics. However, this simplification requires the mem2reg pass, thus mem2reg had to be enabled to work over multiple blocks.

In order to merge slices during flattening, the smaller slice to be merged is filled with placeholder data. This leads to a new challenge though. For example in this snippet:

let slice = [0; 2];
let slice = if x != y {
    slice.push_back(y)
} else {
    slice
};
assert(slice[2] == 0);

We need to prevent the assert from passing in the else case. To prevent accessing elements past the dynamic size of the slice, we now codegen an array length check before every slice ArrayGet. This also required having to codegen the slice size. The structure of slices in SSA has been changed to a tuple (slice length, slice contents), which the length is a field and the contents are an Array in SSA.

This new slice size codegen may also be useful for codegen of dynamic slices as the size of a slice is directly stored and updated during execution of the program.

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.

[vezenovm]

This is ready for review again

[jfecher]

LGTM 👍