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/ir/serializationRegressions/dispatchReceiverValue.kt

@@ -3,7 +3,7 @@
 // !LANGUAGE: +MultiPlatformProjects
 // IGNORE_BACKEND: JS_IR
 // IGNORE_BACKEND: JS_IR_ES6
-// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR
 // FIR status: expect/actual in the same module (ACTUAL_WITHOUT_EXPECT)
 // MODULE: lib
 // FILE: common.kt

compiler/testData/codegen/box/ir/serializationRegressions/signatureClash.kt

@@ -4,7 +4,7 @@
 // IGNORE_BACKEND: JS_IR
 // IGNORE_BACKEND: JS_IR_ES6
 // IGNORE_BACKEND: NATIVE
-// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR
 // FIR status: validation failed. TODO decide if we want to fix KT-42020 for FIR as well
 // MODULE: lib
 // FILE: lib.kt

compiler/testData/codegen/box/ir/serializationRegressions/varAsFunctionCall.kt

@@ -1,4 +1,4 @@
-// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR
 // FIR status: ARGUMENT_TYPE_MISMATCH at ok.z(fail), actual String, expected T
 // MODULE: lib
 // FILE: l1.kt