[perl #47047] Fix erroneous AUTOLOAD warning

[perl #47047] Fix erroneous AUTOLOAD warning

If there was a stub present in the package into which the invocant had
been blessed, then AUTOLOADing via a *method* call would warn with ‘Use
of inherited AUTOLOAD for non-method’ even if it is a method.

A recent commit stopped OPf_REF from being set on OP_ENTERSUB, so this
commit uses that flag to indicate a method call, to allow a fast run-
time check to see whether to pass the method flag to gv_autoload.