Solution for KT-7838: now data flow information at the end of when expression is determined correctly.

#KT-7838  Fixed.

compiler/testData/diagnostics/tests/when/BranchFalseBypass.kt

@@ -0,0 +1,12 @@
+enum class My { A, B }
+
+fun test(a: My): String {
+    val q: String?
+
+    when (a) {
+        My.A -> q = "1"
+        My.B -> q = "2"
+    }
+    // When is exhaustive
+    return <!DEBUG_INFO_SMARTCAST!>q<!>
+}