pgsql: Re-allow INDEX_VAR as rt_index in ChangeVarNodes().

Re-allow INDEX_VAR as rt_index in ChangeVarNodes().

Apparently some extensions are in the habit of calling
ChangeVarNodes() with INDEX_VAR as the rt_index to replace.
That worked before 2489d76c4, at least as long as there were
not PlaceHolderVars in the expression; but now it fails
because bms_is_member spits up.  Add a test to avoid that.

Per report from Anton Melnikov, though this is not his
proposed patch.

Discussion: https://postgr.es/m/5b370a46-f6d2-373d-9dbc-0d55250e82c1@inbox.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/fbf80421ead55deaafbefa808960a2695de492c9

Modified Files
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src/backend/rewrite/rewriteManip.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)