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/kt20651_inlineLambda.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 enum class Test(val x: String, val closure1: () -> String) {
     FOO("O", run { { FOO.x } }) {
         override val y: String = "K"