CFG exhaustive when else instruction for KT-8700

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

@@ -0,0 +1,11 @@
+fun foo(b: Boolean): Int {
+    val x: Int
+    val y: Int
+    when (b) {
+        true -> y = 1
+        false -> y = 0
+    }
+    // x is initialized here
+    x = 3
+    return x + y
+}