pgsql: Properly check index mark/restore in ExecSupportsMarkRestore.

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

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6dda057043df4a56683e8fa31a74737b63da4c47

Modified Files
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src/backend/executor/execAmi.c | 5 +++++
src/backend/optimizer/util/plancat.c | 2 ++
src/include/nodes/pathnodes.h | 1 +
3 files changed, 8 insertions(+)