JVM_IR: Lower IrGetEnumValue to this whenever possible.

The reference can be lowered to `this` if it is captured in the lexical
scope of the corresponding enum entry, and not used by the enum entry's
super constructor. Otherwise, it is lowered to
`GETFIELD SomeEnum.SomeEntry`.

compiler/testData/codegen/box/enum/kt7257_anonObjectInit.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 enum class X {
     B {
         val value2 = "K"