[FIR] Don't create initializers for simple enum entries

Usually FIR enum entry is initialized by anonymous object,
which is the container for all enum entry' declarations.
However, for simple enum entries there is no need of initializer at all.

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

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // CHECK_CASES_COUNT: function=box count=0
 // CHECK_IF_COUNT: function=box count=1
 

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

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // WITH_RUNTIME
 // CHECK_CASES_COUNT: function=foo count=0
 // CHECK_IF_COUNT: function=foo count=3