chore(ssa refactoring): Support basic recursive functions

[jfecher]

Description

Problem

The SSA Refactoring could support recursive functions with minimal changes, but does not.

Summary

Supports basic recursive functions in the ssa refactoring. This was a ~6 line change (all other lines are for the new test). This supports only "basic" recursive functions that are able to be evaluated during inlining because of constant folding. We could make this more complex in the future but this should support the majority of use cases as-is. It is also still worth noting that this recursion must (like loops) terminate during compilation so calling a recursive function like factorial on a private input will still result in hitting the recursion limit.

Example

Before the code:

fn main() -> pub Field {
    factorial(5)
}

fn factorial(x: Field) -> Field {
    if x < 1 {
        1
    } else {
        x * factorial(x - 1)
    }
}

Will hit the recursion limit in the ssa refactoring (likewise in the current ssa we issue an error stating recursive functions are not supported). After this PR it will inline factorial correctly and main will become equivalent to:

fn main() -> pub Field {
    120
}

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.

[joss-aztec]

Nice!