[RegAllocGreedy] Fix an assertion and condition when last chance recoloring is used.

When last chance recoloring is used, the list of NewVRegs may not be
empty when calling selectOrSplitImpl. Indeed, another coloring may have
taken place with splitting/spilling in the same recoloring session.

Relax an assertion to take this into account and adapt a condition to
act as if the NewVRegs were local to this selectOrSplitImpl instance.

Unfortunately I am unable to produce a test case for this, I was only
able to reproduce the conditions on an out-of-tree target.

llvm-svn: 281782