[FIR] Fix wrong ConstantReference for returnsNotNull in EffectExtractor

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/deeplyNested.fir.kt

@@ -68,12 +68,12 @@ fun nested2(x: Any?) {
 
 fun nested3(x: Any?) {
     myAssert(equalsTrue(notEqualsNull(nullWhenNotString(x))))
-    x.<!UNRESOLVED_REFERENCE!>length<!>
+    x.length
 }
 
 fun branchedAndNested(x: Any?, y: Any?) {
     myAssert(equalsTrue(notEqualsNull(nullWhenNotString(x))) && equalsTrue(isString(y)))
-    x.<!UNRESOLVED_REFERENCE!>length<!>
+    x.length
     y.length
 }
 
@@ -97,7 +97,7 @@ fun branchedAndNestedWithNativeOperators(x: Any?, y: Any?) {
             &&
             (1 == 2 || y is Int || isString(y))
     )
-    x.<!UNRESOLVED_REFERENCE!>length<!>
+    x.length
     y.length
     y.inc()
 }