fix: ignore calls to Intrinsic::AsWitness during brillig codegen

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
+}