This is a follow-up to r353988 where tryEvict was extended to take last
chance recoloring into account. Now we do the same thing for trySplit and
Now we always pass a "FixedRegisters" argument to canEvictInterference and
tryEvict so it doesn't need to have a default value anymore.
The need for this was found long ago in an out-of-tree target.
Unfortunately I don't have a reproducer for an in-tree target.