pgsql: Fix power() for large inputs yet more.

Fix power() for large inputs yet more.

Buildfarm results for commit e532b1d57 reveal the error in my thinking
about the unexpected-EDOM case.  I'd supposed this was no longer really
a live issue, but it seems the fix for glibc's bug #3866 is not all that
old, and we still have at least one buildfarm animal (lapwing) with the
bug.  Hence, resurrect essentially the previous logic (but, I hope, less
opaquely presented), and explain what it is we're really doing here.

Also, blindly try to fix fossa's failure by tweaking the logic that
figures out whether y is an odd integer when x is -inf.  This smells
a whole lot like a compiler bug, but I lack access to icc to try to
pin it down.  Maybe doing division instead of multiplication will
dodge the issue.

Discussion: https://postgr.es/m/E1jkU7H-00024V-NZ@gemulon.postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5674eb98762ed41c7f9672a3bc56edf925e9d97d

Modified Files
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src/backend/utils/adt/float.c | 34 +++++++++++++++++++++++-----------
1 file changed, 23 insertions(+), 11 deletions(-)