[RegAllocGreedy] Fix the list of NewVRegs for last chance recoloring.

When trying to recolor a register we may split live-ranges in the
process. When we create new live-ranges we will have to process them,
but when we move a register from Assign to Split, the allocation is not
changed until the whole recoloring session is successful.
Therefore, only push the live-ranges that changed from Assign to
Split when the recoloring is successful.

Same as the previous commit, I was not able to produce a test case that
reproduce the problem with in-tree targets.

Note: The bug has been here since the recoloring scheme has been added
back in r200883 (Feb 2014).

llvm-svn: 281783