Do not throw an exception when a when-statement with no else makes no match

compiler/testData/codegen/box/when/noElseExhaustiveStatement.kt

@@ -0,0 +1,12 @@
+enum class En {
+    A
+    B
+}
+
+fun box(): String {
+    when(En.A) {
+        En.A -> "s1"
+        En.B -> "s2"
+    }
+    return "OK"
+}