Dodge a compiler bug affecting timetz_zone/timetz_izone.

Dodge a compiler bug affecting timetz_zone/timetz_izone.

Use a modulo operator instead of implementing the same behavior
with a loop.  The loop solution is doubtless microscopically
faster for the typical case of only wrapping into the very next
day, but maybe not so much for large interval values.  In any
case, timetz is such a backwater that it's doubtful anybody
would notice any performance change anyway.

This avoids a compiler bug occurring in AIX's xlc, even in pretty
late-model revisions.

We did not have test coverage for the case where the initial
result->time value is negative, so add that.

For the moment, install this only in HEAD.  My plan is to
back-patch the test case, and then the code change assuming that
buildfarm testing proves the bug occurs in the back branches.
(That seems pretty likely, but let's find out for sure.)

Per buildfarm results from commits 97957fdba and 2f0472030.
Thanks to Michael Paquier for the idea to use a modulo operation
to replace the faulty loop.

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