Test infra: refactor IGNORE_BACKEND directive

treat it as a general one, introduce *_K1 and *_K2 variants for
more specific ignoring

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

@@ -1,5 +1,5 @@
 // !LANGUAGE: +EnumEntries
-// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR
 // IGNORE_BACKEND: JS, JVM
 // WITH_STDLIB
 

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

@@ -1,5 +1,5 @@
 // TARGET_BACKEND: JVM
-// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR
 // FIR status: don't support legacy feature (see https://youtrack.jetbrains.com/issue/KT-37591). UNRESOLVED_REFERENCE at '+'
 // WITH_STDLIB
 // MODULE: lib