pgsql: Fix tuplesort optimization for CLUSTER-on-expression.

Fix tuplesort optimization for CLUSTER-on-expression.

When dispatching sort operations to specialized variants, commit
69749243 failed to handle the case where CLUSTER-sort decides not to
initialize datum1 and isnull1. Fix by hoisting that decision up a level
and advertising whether datum1 can be relied on, in the Tuplesortstate
object.

Per reports from UBsan and Valgrind build farm animals, while running
the cluster.sql test.

Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/CAFBsxsF1TeK5Fic0M%2BTSJXzbKsY6aBqJGNj6ptURuB09ZF6k_w%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cc58eecc5d75a9329a6d49a25a6499aea7ee6fd6

Modified Files
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src/backend/utils/sort/tuplesort.c | 78 +++++++++++++++++++++++++++-----------
1 file changed, 56 insertions(+), 22 deletions(-)