feat: Remove an unnecessary witness in mul_with_witness (#2078)

crates/noirc_evaluator/src/ssa_refactor/acir_gen/acir_ir/generated_acir.rs

@@ -260,7 +260,7 @@ impl GeneratedAcir {
     /// If one has multiplicative term and the other is of degree one or more,
     /// the function creates [intermediate variables][`Witness`] accordingly.
     /// There are two cases where we can optimize the multiplication between two expressions:
-    /// 1. If both expressions have at most a total degree of 1 in each term, then we can just multiply them
+    /// 1. If the sum of the degrees of both expressions is at most 2, then we can just multiply them
     /// as each term in the result will be degree-2.
     /// 2. If one expression is a constant, then we can just multiply the constant with the other expression
     ///
@@ -271,10 +271,14 @@ impl GeneratedAcir {
         let lhs_is_linear = lhs.is_linear();
         let rhs_is_linear = rhs.is_linear();
 
-        // Case 1: Both expressions have at most a total degree of 1 in each term
-        if lhs_is_linear && rhs_is_linear {
-            return (lhs * rhs)
-                .expect("one of the expressions is a constant and so this should not fail");
+        // Case 1: The sum of the degrees of both expressions is at most 2.
+        //
+        // If one of the expressions is constant then it does not increase the degree when multiplying by another expression.
+        // If both of the expressions are linear (degree <=1) then the product will be at most degree 2.
+        let both_are_linear = lhs_is_linear && rhs_is_linear;
+        let either_is_const = lhs.is_const() || rhs.is_const();
+        if both_are_linear || either_is_const {
+            return (lhs * rhs).expect("Both expressions are degree <= 1");
         }
 
         // Case 2: One or both of the sides needs to be reduced to a degree-1 univariate polynomial
@@ -288,7 +292,7 @@ impl GeneratedAcir {
         // rhs, we only need to square the lhs.
         if lhs == rhs {
             return (&*lhs_reduced * &*lhs_reduced)
-                .expect("Both expressions are reduced to be degree<=1");
+                .expect("Both expressions are reduced to be degree <= 1");
         };
 
         let rhs_reduced = if rhs_is_linear {
@@ -297,7 +301,7 @@ impl GeneratedAcir {
             Cow::Owned(self.get_or_create_witness(rhs).into())
         };
 
-        (&*lhs_reduced * &*rhs_reduced).expect("Both expressions are reduced to be degree<=1")
+        (&*lhs_reduced * &*rhs_reduced).expect("Both expressions are reduced to be degree <= 1")
     }
 
     /// Signed division lhs /  rhs