Make 'when' on nullable enums exhaustive if 'null' entry is present

#KT-2902 Fixed

compiler/testData/codegen/boxWithStdlib/whenEnumOptimization/nullableEnum.kt

@@ -0,0 +1,14 @@
+enum class E {
+    A
+    B
+}
+
+fun test(e: E?) = when (e) {
+    E.A -> "Fail A"
+    null -> "OK"
+    E.B -> "Fail B"
+}
+
+fun box(): String {
+    return test(null)
+}