FIR. Refactor smart-cast representation in FIR tree

Make smart-casts non-transparent expression without delegation
to underlying FirQualifiedAccessExpression, as children delegation in
fir tree has unclear semantics
Remove two different kinds of tree nodes for smart-casts

analysis/analysis-api/testData/components/callResolver/resolveCall/whenSelectorSmartCast.kt

@@ -0,0 +1,7 @@
+fun test(obj: Any): String {
+    return when {
+        obj !is Iterable<*> -> "not iterable"
+        <expr>obj</expr> !is Collection<*> -> "not collection"
+        else -> "unknown"
+    }
+}