Make left-join removal safe under -DREALLOCATE_BITMAPSETS.

Make left-join removal safe under -DREALLOCATE_BITMAPSETS.

The initial building of RestrictInfos and SpecialJoinInfos tends to
create structures that share relid sets (such as syn_lefthand and
outer_relids).  There's nothing wrong with that in itself, but when
we modify those relid sets during join removal, we have to be sure
not to corrupt the values that other structures are pointing at.
remove_rel_from_query() was careless about this.  It accidentally
worked anyway because (1) we'd never be reducing the sets to empty,
so they wouldn't get pfree'd; and (2) the in-place modification is the
same one that we did or will apply to the other struct's relid set,
so that there wasn't visible corruption at the end of the process.

While there's no live bug in a standard build, of course this is way
too fragile to accept going forward.  (Maybe we should back-patch
this change too for safety, but I've refrained for now.)

This bug was exposed by the recent invention of REALLOCATE_BITMAPSETS.
Commit e0477837c had installed a fix, but that went away with the
reversion of SJE, so now we need to fix it again.

David Rowley and Tom Lane

Discussion: https://postgr.es/m/CACJufxFVQmr4=JWHAOSLuKA5Zy9H26nY6tVrRFBOekHoALyCkQ@mail.gmail.com