Added is operator

src/main/java/chocopy/pa3/CodeGenImpl.java

@@ -728,7 +728,17 @@ public class CodeGenImpl extends CodeGenBase
                 sp_off -= 2;
                 backend.emitADDI(SP, FP, -sp_off*wordSize, "restore sp");
             }
-            // TODO: Implement "is" for objects
+            else if(operator.equals("is"))
+            {
+                node.right.dispatch(this);
+                backend.emitSW(Register.A0, Register.FP, -sp_off*wordSize, "Push on stack slot "+sp_off);
+                incSp(1);
+                node.left.dispatch(this);
+                sp_off--;
+                backend.emitLW(Register.T0, Register.FP, -sp_off*wordSize, "Pop stack slot "+sp_off);
+                backend.emitXOR(Register.A0,  Register.A0, Register.T0, "Operator: is");
+                backend.emitSEQZ(Register.A0,  Register.A0, "Result is True if XOR equals 0");
+            }
             else
             {
                 backend.emitJAL(errorNI, "Operator not implemented");