fix(ssa refactor): Reset condition value during flattening pass

[jfecher]

Description

Problem*

Resolves #1792 / #1806

Summary*

When flattening the cfg there is an optimization to set the value of an if's condition to a constant true/false within the then/else branch respectively since there is no other way to get to these branches without this being true.

This means for code such as:

fn main(c: bool) {
    if c {
        constrain c;
    }
}

We can optimize it to:

fn main(c: bool) {
    if c {
        constrain true;
    }
}

Since we know c is always true within the then branch. This allows us to remove this particular constrain entirely afterward.

The problem in #1792 arised when we forgot to reset the condition c back to c, so even after the if ended we still had c = true or c = false in the context. This PR fixes this by simply resetting c to its old value after finishing an if statement.

There is another, related fix in this PR that we now avoid setting c = true or c = false if c is already a known constant. If this happened, we could be setting true = false or false = true which would not be good.

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]

@ludamad do you think I should include the integration test 1792_bad_equality from #1806 in this PR? I thought it was a bit repetitive to include all 3 of the tests in that PR for the same issue so I only included the smallest one initially.

[ludamad]

LGTM