rpeep(): OP_SORT with code block has OPf_SPECIAL

rpeep(): OP_SORT with code block has OPf_SPECIAL

In rpeep(), we check whether the OP_SORT has a code block as the first arg
(e.g. sort {BLOCK} ...) by testing for OPf_STACKED, then looking for
an OP_SCOPE or ex-OP_LEAVE. However, ck_sort() has already checked for
this situation and set the OPf_SPECIAL flag. So just just check for this
flag in  rpeep(); leave in the OP_SCOPE/ex-OP_LEAVE checks as just
assertions.

Also, add some commentary to ck_sort() and S_simplify_sort().