fix: ignore calls to Intrinsic::AsWitness during brillig codegen (n…

…oir-lang#5350)

# Description

## Problem\*

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

## Summary\*

This PR addresses an issue where if a call is made to `std::as_witness`
from a brillig runtime then the compiler will panic with the unreachable
error directly below where the new match arm is added.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_block.rs

@@ -583,6 +583,11 @@ impl<'block> BrilligBlock<'block> {
                         1,
                     );
                 }
+
+                // `Intrinsic::AsWitness` is used to provide hints to acir-gen on optimal expression splitting.
+                // It is then useless in the brillig runtime and so we can ignore it
+                Value::Intrinsic(Intrinsic::AsWitness) => (),
+
                 _ => {
                     unreachable!("unsupported function call type {:?}", dfg[*func])
                 }

test_programs/execution_success/as_witness/Nargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "as_witness"
+type = "bin"
+authors = [""]
+
+[dependencies]

test_programs/execution_success/as_witness/Prover.toml

@@ -0,0 +1 @@
+a = 42

test_programs/execution_success/as_witness/src/main.nr

@@ -0,0 +1,5 @@
+// Simple example of checking where two arrays are different
+fn main(a: Field) -> pub Field {
+    std::as_witness(a);
+    a
+}