Duplicated code with --experimental-ssa flattening pass

[jfecher]

Aim

Tried to use an if statement without an else:

fn main(x: bool) {
    if x {
        dep::std::println(5);
    }
    dep::std::println(6);
}

Expected Behavior

Expected the following resulting IR

fn main f1 {
  b0(v0: u1):
    v9 = call println(Field 5)
    v10 = not v0
    v12 = call println(Field 6)
    return unit 0
}

Bug

The flatten-cfg pass duplicates every instruction in the end block:

fn main f1 {
  b0(v0: u1):
    v9 = call println(Field 5)
    v10 = not v0
    v12 = call println(Field 6)
    v13 = call println(Field 6)
    return unit 0
}

To Reproduce

Installation Method

None

Nargo Version

No response

Additional Context

No response

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