Fix bitshiftright()'s zero-padding some more.

Commit 5ac0d936 failed to entirely fix bitshiftright's habit of
leaving one-bits in the pad space that should be all zeroes,
because in a moment of sheer brain fade I'd concluded that only
the code path used for not-a-multiple-of-8 shift distances needed
to be fixed.  Of course, a multiple-of-8 shift distance can also
cause the problem, so we need to forcibly zero the extra bits
in both cases.

Per bug #16037 from Alexander Lakhin.  As before, back-patch to all
supported branches.

Discussion: https://postgr.es/m/16037-1d1ebca564db54f4@postgresql.org