Analyze Data Flow: Initial implementation

#KT-11994 In Progress

idea/testData/slicer/inflow/whenExpression.kt

@@ -0,0 +1,9 @@
+// FLOW: IN
+
+fun test(m: Int, n: Int) {
+    val <caret>x = when (m) {
+        1 -> 1
+        2 -> n
+        else -> 0
+    }
+}