fix(acvm): Mark outputs of Opcode::Call solvable (#4708)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

No issue as bug found while experimenting

## Summary\*

We should mark the outputs of a call as solvable. Otherwise we can
potentially have more than one linear term and the outputs of call will
not be used by the transformers.

The new test `fold_call_witness_condition` would previously error out
with the following when `nargo info` was ran:
```
The backend encountered an error: "backend_binary: /home/runner/work/aztec-packages/aztec-packages/barretenberg/cpp/src/barretenberg/dsl/acir_format/acir_to_constraint_buf.hpp:57: poly_triple acir_format::serialize_arithmetic_gate(const Program::Expression &): Assertion `(arg.mul_terms.size() <= 1)' failed.\n"
```
Now it successfully calls nargo info on main (we still need to update
fetching the info for every circuit).

## Additional Context



## 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](https://prettier.io/)
and/or `cargo fmt` on default settings.

acvm-repo/acvm/src/compiler/transformers/mod.rs

@@ -142,7 +142,11 @@ pub(super) fn transform_internal(
                 new_acir_opcode_positions.push(acir_opcode_positions[index]);
                 transformed_opcodes.push(opcode);
             }
-            Opcode::Call { .. } => {
+            Opcode::Call { ref outputs, .. } => {
+                for witness in outputs {
+                    transformer.mark_solvable(*witness);
+                }
+
                 // `Call` does not write values to the `WitnessMap`
                 // A separate ACIR function should have its own respective `WitnessMap`
                 new_acir_opcode_positions.push(acir_opcode_positions[index]);

test_programs/execution_success/fold_call_witness_condition/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "fold_call_witness_condition"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.26.0"
+
+[dependencies]

test_programs/execution_success/fold_call_witness_condition/Prover.toml

@@ -0,0 +1,5 @@
+# TODO(https://github.com/noir-lang/noir/issues/4707): Change these inputs to fail the assertion in `fn return_value`
+# and change `enable` to false. For now we need the inputs to pass as we do not handle predicates with ACIR calls
+x = "5"
+y = "10"
+enable = true

test_programs/execution_success/fold_call_witness_condition/src/main.nr

@@ -0,0 +1,16 @@
+global NUM_RESULTS = 2;
+fn main(x: Field, y: pub Field, enable: bool) -> pub [Field; NUM_RESULTS] {
+    let mut result = [0; NUM_RESULTS];
+    for i in 0..NUM_RESULTS {
+        if enable {
+            result[i] = return_value(x, y);
+        }
+    }
+    result
+}
+
+#[fold]
+fn return_value(x: Field, y: Field) -> Field {
+    assert(x != y);
+    x
+}

tooling/debugger/ignored-tests.txt

@@ -14,3 +14,4 @@ signed_comparison
 to_bytes_integration
 fold_basic
 fold_basic_nested_call
+fold_call_witness_condition