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

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 8fbdd63a52df670273990e126f1ab7895c14978a
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2018-05-02T21:32:40Z
Releases: 9.5.13
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

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