[RegAllocGreedy]: Allow recoloring of done register if it's non-tied

Summary:
If we have a non-allocated register, we allow us to try recoloring of an
already allocated and "Done" register, even if they are of the same
register class, if the non-allocated register has at least one tied def
and the allocated one has none.

It should be easier to recolor the non-tied register than the tied one, so
it might be an improvement even if they use the same regclasses.

Reviewers: qcolombet

Reviewed By: qcolombet

Subscribers: llvm-commits, MatzeB

Differential Revision: https://reviews.llvm.org/D38309

llvm-svn: 314388