Add a RAGreedy::canEvict function.

This doesn't change functionality (much), but it allows for a more fine-grained
eviction policy. The current policy only compares spill weights, and that is not
always the best thing to do.  Spill weights are designed to serve linear scan,
and they don't consider live range splitting.

Add a mechanism so canEvict() can request that a live range be evicted and
split/spilled. This is to avoid infinite eviction loops.

llvm-svn: 132101