[FIR] Fix generating this reference in delegated accessors

There was a problem with delegated extension property with dispatch
  receiver that `this` in `getValue` call was set to dispatch receiver
  instead of extension one

compiler/testData/codegen/box/delegatedProperty/provideDelegate/memberExtension.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // WITH_RUNTIME
 
 object Host {
@@ -13,4 +12,4 @@ object Host {
     val ok = "O".plusK
 }
 
-fun box(): String = Host.ok
+fun box(): String = Host.ok