Regexp replaces using large replacement variables fail some of

Actually the patch wasn't good, since it marked as
non-constant too many
types of substitutions.
So I reverted it and committed another fix instead:

Change 29617 on 2006/12/23 by rgsmarais

        A better fix for RT #6006: revert change 29616,
which was bogus,
        and add OP_SCOPE to the list of
"dangerous" ops that might appear
        on the right side of s///.
        However, this fix does not work under taint mode,
and op/subst.t
        has -T.

I added a corresponding TODO test (this bug is still present
with -T).