IR KT-50258 rewrite enum-based 'when' more carefully

compiler/testData/codegen/box/when/enumOptimization/kt50258.kt

@@ -0,0 +1,23 @@
+enum class MyEnum {
+    First,
+    Second
+}
+
+fun getValue() = MyEnum.First
+
+var result = "Failed"
+
+fun getLambda(): (Int) -> Unit =
+    when (val value = getValue()) {
+        MyEnum.Second -> { _ -> }
+        MyEnum.First -> { _ ->
+            if (value == MyEnum.First) {
+                result = "OK"
+            }
+        }
+    }
+
+fun box(): String {
+    getLambda().invoke(2)
+    return result
+}