FIR DFA: align FirLocalVariableAssignmentAnalyzer with the graph builder

I.e. maintain a set of seen lambdas where each marked as either "data
flow only" or "both data and control flow". The latter are truly
parallel with the function being analyzed, while the former have
technically already terminated, we just don't know the types inside them
because they may not have been analyzed yet.

compiler/testData/diagnostics/testsWithStdLib/smartcasts/lambdaInCallArgs.kt

@@ -10,7 +10,7 @@ fun test_1() {
         foo(
             <!DEBUG_INFO_SMARTCAST!>x<!>.length, // stable smartcast
             run { x = "" },
-            <!SMARTCAST_IMPOSSIBLE!>x<!>.length  // stable smartcast
+            <!SMARTCAST_IMPOSSIBLE!>x<!>.length  // can be stable smartcast
         )
     }
 }
@@ -36,3 +36,14 @@ fun test_3() {
         )
     }
 }
+
+fun test_4() {
+    var x: String? = null
+    if (x != null) {
+        foo(
+            <!DEBUG_INFO_SMARTCAST!>x<!>.length, // stable smartcast
+            run { x = null },
+            <!SMARTCAST_IMPOSSIBLE!>x<!>.length  // either unstable or not a smartcast
+        )
+    }
+}