Remove accidental hack in ipa_polymorphic_call_context::set_by_invariant

Remove accidental hack in ipa_polymorphic_call_context::set_by_invariant

I managed to commit a hack setting offset to 0 in
ipa_polymorphic_call_context::set_by_invariant.  This makes it to give up on multiple
inheritance, but most likely won't give bad code since the ohter base will be of
different type.

gcc/ChangeLog:

	* ipa-polymorphic-call.cc (ipa_polymorphic_call_context::set_by_invariant): Remove
	accidental hack reseting offset.