fix(ssa): Keep defaults for values returned in the databus

[aakoshh]

Description

Problem*

Resolves #9903

Summary*

Make sure that any value returned in the databus is not removed from SSA:

• remove_uncreachable_instruction will replace them with defaults even if the block is Unreachable and all other instructions are removed.
• dead_instruction_elimination will consider these instructions used, even if their results are not used by anything else, because they stop appearing in the return terminator of an unreachable block.

Used #9991 to add unit tests, because integration tests don't cover SSA.

Additional Context

I tested with a program like this:

global G_A: [u32; 2] = [1, 2];
fn main(b: bool) -> return_data [u32; 2] {
    if b {
        let _a = G_A[2];
    } else {
        let _a = G_A[3];
    }
    G_A
}

I expected this to result in a CFG like b0 -> [b1, b2] -> b3, where b1 and b2 both become unreachable, and then b3, where the return_data is created, would not even appear in reachable_blocks(), and leaving the value in the databus unmapped, leading to a panic. Lucky for us: 1) remove_unreachable_instruction runs on the flattened CFG in ACIR and 2) in Brillig the return_data becomes None; otherwise this would be a problem.

In our case we have this SSA:

After Simplifying (4) (step 33):
g0 = u32 1
g1 = u32 2
g2 = make_array [u32 1, u32 2] : [u32; 2]

acir(inline) predicate_pure fn main f0 {
  b0(v3: u1):
    constrain u1 0 == v3, "Index out of bounds"
    constrain u1 0 == u1 1, "Index out of bounds"
    enable_side_effects u1 1
    v8 = make_array [Field 1, Field 2] : [Field; 2]
    return v8
}

After Remove Unreachable Instructions (1) (step 34):
g0 = u32 1
g1 = u32 2
g2 = make_array [u32 1, u32 2] : [u32; 2]

acir(inline) predicate_pure fn main f0 {
  b0(v3: u1):
    constrain u1 0 == v3, "Index out of bounds"
    constrain u1 0 == u1 1, "Index out of bounds"
    v7 = make_array [Field 0, Field 0] : [Field; 2]
    unreachable
}

So the change ensures that v7, which is what assigns the value to the returned Field array, still gets created, even though it's never returned.

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

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

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'ACVM Benchmarks'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 7b88baf	Previous: 39f193c	Ratio
perfectly_parallel_batch_inversion_opcodes	2781693 ns/iter (± 1335)	2254940 ns/iter (± 1115)	1.23

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Test Suite Duration'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 7b88baf	Previous: 39f193c	Ratio
test_report_zkpassport_noir-ecdsa_	3 s	2 s	1.50

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench