JVM_IR: fix NPE in interface companion initializers

* When referencing the companion itself, they should use the $$INSTANCE
  field, not the (null until <clinit> returns) Companion field of the
  interface.

* Interface companion init blocks should be made static.

compiler/testData/codegen/box/objects/interfaceCompanion.kt

@@ -1,5 +1,4 @@
 // IGNORE_BACKEND_FIR: JVM_IR
-// IGNORE_BACKEND: JVM_IR
 // Inside of the companion we have to access the instance through the local Companion field,
 // not by indirection through the Companion field of the enclosing class.
 // Class initialization might not have finished yet.

compiler/testData/codegen/box/objects/interfaceCompanionObjectReference.kt

@@ -1,6 +1,5 @@
 // !LANGUAGE: +NestedClassesInAnnotations
 // IGNORE_BACKEND_FIR: JVM_IR
-// IGNORE_BACKEND: JVM_IR
 // WITH_RUNTIME
 
 import kotlin.test.*

compiler/testData/codegen/box/objects/selfReferenceToInterfaceCompanionObjectInInlineLambdaInConstructorBody.kt

@@ -1,5 +1,4 @@
 // IGNORE_BACKEND_FIR: JVM_IR
-// IGNORE_BACKEND: JVM_IR
 interface Test {
     companion object {
         fun ok() = "OK"