Add RAGreedy::calcCompactRegion.

This method computes the edge bundles that should be live when splitting
around a compact region. This is independent of interference.

The function returns false if the live range was already a compact
region, or the compact region doesn't have any live bundles - it would
be the same as splitting around basic blocks.

Compact regions are computed using the normal spill placement code. We
pretend there is interference in all live-through blocks that don't use
the live range. This removes all edges from the Hopfield network used
for spill placement, so it converges instantly.

llvm-svn: 135847