Pass DataFlowInfo through when-conditions

compiler/testData/diagnostics/tests/dataFlowInfoTraversal/When.jet

@@ -11,7 +11,7 @@ fun foo() {
     }
 
     when (x) {
-        0 -> { if (x == null) return }
+        0 -> { if (<!SENSELESS_COMPARISON!>x == null<!>) return }
         else -> { if (x == null) return }
     }
     bar(x)

compiler/testData/diagnostics/tests/regressions/Jet169.jet

@@ -1,7 +1,7 @@
 fun set(<!UNUSED_PARAMETER!>key<!> : String, <!UNUSED_PARAMETER!>value<!> : String) {
   val a : String? = ""
   when (a) {
-    "" -> a<!UNSAFE_CALL!>.<!>get(0)
+    "" -> a.get(0)
     is String, is Any -> a.compareTo("")
     else -> a.toString()
   }