FIR2IR: Fix substitution work for context receivers

compiler/testData/codegen/box/extensionClasses/genericCollection.kt

@@ -1,6 +1,5 @@
 // !LANGUAGE: +ContextReceivers
 // TARGET_BACKEND: JVM_IR
-// IGNORE_BACKEND_FIR: JVM_IR
 // FIR status: context receivers aren't yet supported
 // WITH_STDLIB
 
@@ -12,4 +11,4 @@ fun box(): String {
     with (listOf(1, 2, 3)) {
         return A().result
     }
-}
+}

compiler/testData/codegen/box/extensionClasses/generics.kt

@@ -1,6 +1,5 @@
 // !LANGUAGE: +ContextReceivers
 // TARGET_BACKEND: JVM_IR
-// IGNORE_BACKEND_FIR: JVM_IR
 // FIR status: context receivers aren't yet supported
 
 context(T) class B<T : CharSequence> {
@@ -9,4 +8,4 @@ context(T) class B<T : CharSequence> {
 
 fun box() = with("OK") {
     B().result
-}
+}