Properly check index mark/restore in ExecSupportsMarkRestore.
Previously this code assumed that all IndexScan nodes supported mark/restore, which is not true since it depends on optional index AM support functions. This could lead to errors about missing support functions in rare edge cases of mergejoins with no sort keys, where an unordered non-btree index scan was placed on the inner path without a protecting Materialize node. (Normally, the fact that merge join requires ordered input would avoid this error.) Backpatch all the way since this bug is ancient. Per report from Eugen Konkov on irc. Discussion: https://postgr.es/m/87o8jn50be.fsf@news-spur.riddles.org.uk
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