Support isLit for Property types. (#3782)

Since we override litOption to always be None, we need to override
isLit to check the Binding.

core/src/main/scala/chisel3/properties/Property.scala

@@ -246,6 +246,16 @@ sealed trait Property[T] extends Element { self =>
   }
 
   override def litOption: Option[BigInt] = None
+
+  /** Return whether this Property is a literal.
+    *
+    * Since we override litOption to always be None, we need to override this method to check the Binding.
+    */
+  override def isLit: Boolean = topBindingOpt match {
+    case Some(PropertyValueBinding) => true
+    case _                          => false
+  }
+
   def toPrintable: Printable = {
     throwException(s"Properties do not support hardware printing" + this._errorContext)
   }

src/test/scala/chiselTests/properties/PropertySpec.scala

@@ -607,4 +607,14 @@ class PropertySpec extends ChiselFlatSpec with MatchesAndOmits {
       "propassign flatModule.prop.int, Integer(1)"
     )()
   }
+
+  it should "support isLit" in {
+    ChiselStage.emitCHIRRTL(new RawModule {
+      val port = IO(Input(Property[Int]()))
+      val lit = Property(1)
+
+      port.isLit shouldBe false
+      lit.isLit shouldBe true
+    })
+  }
 }