avoid adding two OP_HINTSEVAL ops

avoid adding two OP_HINTSEVAL ops

Under argless eval (i.e. defaults to $_) in the presence of hints,
the perl core was recursively calling ck_eval, which had the side-effect
of attempting to append an OP_HINTSEVAL op twice. Formerly due to the way
the appending was done, the first OP_HINTSEVAL op was silently discarded;
after doing things "properly" with op_sibling_splice(), both ops ended up
being appended, and at run-time, the hints hash would be interpreted as
the SV to eval.

This wasn't tested in the core tests, and just ended up sometimes failing
warnings.t or Digest-SHA tests (but never on my system!)