Suggestion for a fix to Bug middle-end/20177

[Richard Kenner]

    This is interesting, so there could be cases were want to copy CC
    register when doing SMS.  what happens if we want to move the set of a
    CC to another iteration of the loop ? or the use of the CC ?  but
    usually this is couldn't happen in a simple loop, right? the use of CC
    is eventually used in a branch, or there is something that I am
    missing ?

Basically, you can't move one half of the set/use CC.  That's one of the
reasons why very few (any?) machines use CC0 anymore.