disable ivopts by default for 4.0?

[Zdenek Dvorak]

Hello,

> It seems to be the cause of a lot of the remaining 4.0 regressions, including
> many of the wrong-code regressions.

I think I fixed all wrong-code regressions for ivopts (in particular,
none of the PR's you have gathered in PR18693 is a misscompilation).  If
you have pointers for some not fixed (and not with pending patch),
please let me know.

In particular, disabling ivopts would "fix" exactly two of the PR's gathered under
the meta-PR:

PR 18595: serious, but not critical, compile-time problems with ivopts.
PR 18048: 10% performance regression on mgrid

And partially help with

PR 18549: Code size increase.  It would probably make sense to disable
  ivopts at -0s for now.

For the remaining PR's under the meta-bug, the part of problems in ivopts
is already fixed.

Under these circumstances, I do not see how disabling ivopts could
somehow dramatically improve situation.

Zdenek