[FIR] Add lValueTypeRef to FirVariableAssignment, fix tests with ASSIGNMENT_TYPE_MISMATCH

compiler/testData/diagnostics/tests/Properties.fir.kt

@@ -2,7 +2,7 @@ var x : Int = 1 + x
    get() : Int = 1
    set(value : <!WRONG_SETTER_PARAMETER_TYPE!>Long<!>) {
       field = value.toInt()
-      field = 1.toLong()
+      field = <!ASSIGNMENT_TYPE_MISMATCH!>1.toLong()<!>
     }
 
  val xx : Int = <!PROPERTY_INITIALIZER_NO_BACKING_FIELD!>1 + x<!>
@@ -19,4 +19,4 @@ class Test() {
         set(x) {a = x; field = x}
 
    public val i = 1
-}
+}