RegAllocGreedy: Roll back successful recolorings on failure

This is a replacement for the original fix attempted in
c46aab01.

This fixes "overlapping insert" assertion failures when trying to
unwind an unsuccessful recoloring attempt.

The problem would occur when there are multiple recoloring candidates
which recursively required recoloring. If one recoloring candidate was
successfully recolored at one level, and the next recoloring candidate
was unsuccessful, we would not roll back the first candidates
successful recoloring. The forgotten successful recoloring may have
been assigned to something that conflicts with a register that needs
to be restored in a parent recoloring attempt.

See the testcase added in issue48473 for a more concrete example with
explanation.