pgsql: Dodge a compiler bug affecting timetz_zone/timetz_izone.

Dodge a compiler bug affecting timetz_zone/timetz_izone.

This avoids a compiler bug occurring in AIX's xlc, even in pretty
late-model revisions.  Buildfarm testing has now confirmed that
only 64-bit xlc is affected.  Although we are contemplating
dropping support for xlc in v17, it's still supported in the
back branches, so we need this fix.

Back-patch of code changes from HEAD commit 19fa97731.
(The test cases were already back-patched, in 4a427b82c et al.)

Discussion: https://postgr.es/m/CA+hUKGK=DOC+hE-62FKfZy=Ybt5uLkrg3zCZD-jFykM-iPn8yw@mail.gmail.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1ad3a7bd396a802566239fa37e3435df20603d8b

Modified Files
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src/backend/utils/adt/date.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)