Return implementation defined value if pg_$op_s$bit_overflow overflows.

Some older compilers otherwise sometimes complain about undefined
values, even though the return value should not be used in the
overflow case.  We assume that any decent compiler will optimize away
the unnecessary assignment in performance critical cases.

We do not want to restrain the returned value to a specific value,
e.g. 0 or the wrapped-around value, because some fast ways to
implement overflow detecting math do not easily allow for
that (e.g. msvc intrinsics).  As the function documentation already
documents the returned value in case of intrinsics to be
implementation defined, no documentation has to be updated.

Per complaint from Tom Lane and his buildfarm member prairiedog.

Author: Andres Freund
Discussion: https://postgr.es/m/18169.1513958454@sss.pgh.pa.us

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