Fixes for KT-4991: Prefix/postfix expressions resolved to local extensions are not supported in codegen;

KT-4989: Verify error on local functions and extensions;
KT-4987: NegativeArraySizeException on local extension convention function on nullable Int

  #KT-4991 Fixed
  #KT-4989 Fixed
  #KT-4987 Fixed

compiler/testData/codegen/box/operatorConventions/kt4987.kt

@@ -0,0 +1,6 @@
+fun box(): String {
+    fun Int?.inc() = (this ?: 0) + 1
+    var counter: Int? = null
+    counter++
+    return if (counter == 1) "OK" else "fail: $counter"
+}