pgsql: Revert back-branch changes in power()'s behavior for NaN inputs.

Revert back-branch changes in power()'s behavior for NaN inputs.

Per discussion, the value of fixing these bugs in the back branches
doesn't outweigh the downsides of changing corner-case behavior in
a minor release. Hence, revert commits 217d8f3a1 and 4d864de48 in
the v10 branch and the corresponding commits in 9.3-9.6.

Discussion: https://postgr.es/m/75DB81BEEA95B445AE6D576A0A5C9E936A73E741@BPXM05GP.gisp.nec.co.jp

Branch
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REL9_3_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/99095b6320949a0c065bfa381b2d8006ca13e1b5

Modified Files
--------------
src/backend/utils/adt/float.c | 21 +------------
.../expected/float8-exp-three-digits-win32.out | 36 ----------------------
src/test/regress/expected/float8-small-is-zero.out | 36 ----------------------
.../regress/expected/float8-small-is-zero_1.out | 36 ----------------------
src/test/regress/expected/float8.out | 36 ----------------------
src/test/regress/sql/float8.sql | 6 ----
6 files changed, 1 insertion(+), 170 deletions(-)