[LL API] Fix smart-cast collection for special 'FirElement's

'DFANode' is not created for certain elements, such as types or
references. Before the change, only the target element itself was
queried for DFA, making the 'ContextCollector' return an empty map of
smart casts.

^KTIJ-26973 Fixed

analysis/low-level-api-fir/testData/contextCollector/smartCasts/onReference.kt

@@ -0,0 +1,13 @@
+fun test(a: Any) {
+    if (a !is Foo) {
+        return
+    }
+
+    <expr>check</expr>(a.process())
+}
+
+fun check(condition: Boolean) {}
+
+interface Foo {
+    fun process(): Boolean
+}