[FIR] Handle receiver types of extension methods when computing maximally specific types.

Test case (from testPrimitiveReceiver):

fun Short.foo() = 3
fun Int.foo() = 4

1::foo

compiler/testData/codegen/box/callableReference/bound/boundReferenceToOverloadedFunction.kt

@@ -1,5 +1,4 @@
 // !LANGUAGE: +NewInference
-// IGNORE_BACKEND_FIR: JVM_IR
 
 interface JsonParser
 interface JsonCodingParser : JsonParser