CFG exhaustive when else instruction for KT-8700

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

@@ -0,0 +1,22 @@
+fun foo(a: Boolean, b: Boolean): Int {
+    var x: Int
+    if (a) {
+        x = 1
+    }
+    when (b) {
+        true -> x = 2
+        false -> x = 3
+    }
+    return x
+}
+
+fun bar(a: Boolean, b: Boolean): Int {
+    var x: Int
+    if (a) {
+        x = 1
+    }
+    when (b) {
+        false -> x = 3
+    }
+    return <!UNINITIALIZED_VARIABLE!>x<!>
+}