op.c:ck_sort: Restore HINT_LOCALIZE_HH check

I remove this by mistake in commit 354dd559d99.  It makes no
difference to the behaviour.  This check is just for efficiency.
op.c:S_simplify_sort: remove redundant OPf_STACKED check

S_simplify_sort is only called from one spot and only when the
OPf_STACKED flag is not set.
Added example usage and SEE ALSO links to similar modules in doc for if.pm
Increase $if::VERSION to 0.0603