Introduce Value::as_i32

To help make #596 easier to
read and reason about.

ceno_zkvm/src/instructions/riscv/mul.rs

@@ -432,9 +432,7 @@ impl<E: ExtensionField> Signed<E> {
             lkm,
             *val.as_u16_limbs().last().unwrap() as u64,
         )?;
-        let signed_val = val.as_u32() as i32;
-
-        Ok(signed_val)
+        Ok(val.as_i32())
     }
 
     pub fn expr(&self) -> Expression<E> {

ceno_zkvm/src/uint.rs

@@ -684,6 +684,11 @@ impl<'a, T: Into<u64> + From<u32> + Copy + Default> Value<'a, T> {
         self.as_u64() as u32
     }
 
+    /// Convert the limbs to a u32 value
+    pub fn as_i32(&self) -> i32 {
+        self.as_u32() as i32
+    }
+
     pub fn u16_fields<F: SmallField>(&self) -> Vec<F> {
         self.limbs.iter().map(|v| F::from(*v as u64)).collect_vec()
     }