[RAGreedy] Empty live-ranges always succeed in last chance recoloring.

Relax the constraint for empty live-ranges while doing last chance
recoloring. Indeed, those live-ranges do not need an actual color to be
fond for the recoloring to work.
Empty live-range may happen as a result of splitting/spilling.

Unfortunately no test case for in-tree targets.

llvm-svn: 284152