pgsql: Shave a few cycles in compare_pathkeys() by checking for

Log Message:
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Shave a few cycles in compare_pathkeys() by checking for pointer-identical
input lists before we grovel through the lists.  This doesn't save much,
but testing shows that the case of both inputs NIL is common enough that
it saves something.  And this is used enough to be a hotspot.

Modified Files:
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    pgsql/src/backend/optimizer/path:
        pathkeys.c (r1.96 -> r1.97)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/pathkeys.c?r1=1.96&r2=1.97)