[FIR] Introduce the proper Fir2IrResultsConverter

Sometimes when running MPP tests we may
observe js-specific modules running with
the jvm-specific fir2ir converter
(probably because the name didn't contain
the JVM affix).

compiler/testData/ir/irText/classes/cloneable.kt

@@ -1,9 +1,6 @@
 // FIR_IDENTICAL
 // SKIP_KLIB_TEST
-// IGNORE_BACKEND_K1: JS_IR
-// IGNORE_BACKEND_K1: JS_IR_ES6
-// IGNORE_BACKEND_K2: JS_IR
-// IGNORE_BACKEND_K2: JS_IR_ES6
+// IGNORE_BACKEND: JS_IR, JS_IR_ES6
 // STATUS: This should not work in JS, Cloneable is JVM-specific API
 
 class A : Cloneable