Revert parts of c31c291..96b6b87

Revert parts of c31c291..96b6b87

This restores the old definition of dPOPTOPiirl_nomg from
before 96b6b87 and the old definition of dPOPXiirl_ul_nomg from
before e62ca0f (except for a bug fix: POPi cannot be used since
it’s magical). It also reverts most of c31c291.

This does mean that uninitialized warnings for various operators are
back in reverse order. So I am reinstating a bug with this commit. But
that bug was never a 5.14 blocker and so should never have been fixed
during code freeze (and there is the slight possibility that the fix
would break sensitive test suites). It was only fixed ‘for free’ as a
side effect of fixing [perl #87708], but that bug turned out to have a
better fix (commit 75ea7a1) that allows these changes to be reverted.