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/binaryOp/boxingOfLiteralReceiverWithIntegerValueType.kt

@@ -1,6 +1,6 @@
 // !LANGUAGE: -ApproximateIntegerLiteralTypesInReceiverPosition
 // TARGET_BACKEND: JVM
-// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR
 // FIR status: don't support legacy feature
 // WITH_STDLIB
 // This test exists only to check that we don't accidentally break the buggy behavior of the old JVM backend in JVM IR (KT-42321).

compiler/testData/codegen/box/binaryOp/call.kt

@@ -1,5 +1,5 @@
 // LANGUAGE: -ApproximateIntegerLiteralTypesInReceiverPosition
-// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR
 // FIR status: don't support legacy feature; for reasons this test is ignored, go to KT-46419
 
 fun box(): String {

compiler/testData/codegen/box/binaryOp/callNullable.kt

@@ -1,5 +1,5 @@
 // LANGUAGE: -ApproximateIntegerLiteralTypesInReceiverPosition
-// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR
 // FIR status: don't support legacy feature; for reasons this test is ignored, go to KT-46419
 
 fun box(): String {

compiler/testData/codegen/box/binaryOp/intrinsic.kt

@@ -1,5 +1,5 @@
 // LANGUAGE: -ApproximateIntegerLiteralTypesInReceiverPosition
-// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR
 // FIR status: don't support legacy feature; for reasons this test is ignored, go to KT-46419
 
 fun box(): String {

compiler/testData/codegen/box/binaryOp/intrinsicNullable.kt

@@ -1,5 +1,5 @@
 // LANGUAGE: -ApproximateIntegerLiteralTypesInReceiverPosition
-// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR
 // FIR status: don't support legacy feature; for reasons this test is ignored, go to KT-46419
 
 fun box(): String {