Use accessible get/set arguments for collection element receiver

When generating collection element receiver (such as 'a[i]'), accessible
descriptor for get/set operator should be used.
Otherwise, if the corresponding get/set operator fun is called via an
accessor, its argument types may be different in case of generic fun
specialized with primitive types.

 #KT-20387 Fixed

compiler/testData/codegen/box/operatorConventions/kt20387.kt

@@ -0,0 +1,17 @@
+// FILE: test.kt
+import base.*
+
+class Derived : Base<Long>() {
+    inner class Inner {
+        fun foo() = this@Derived[0L]
+    }
+}
+
+fun box() = Derived().Inner().foo()
+
+// FILE: Base.kt
+package base
+
+open class Base<K> {
+    protected operator fun get(key: K) = "OK"
+}