[RAGreedy] Add an experimental deferred spilling feature.

The idea of deferred spilling is to delay the insertion of spill code until the
very end of the allocation. A "candidate" to spill variable might not required
to be spilled because of other evictions that happened after this decision was
taken. The spirit is similar to the optimistic coloring strategy implemented in
Preston and Briggs graph coloring algorithm.

For now, this feature is highly experimental. Although correct, it would require
much more modification to properly model the effect of spilling.

Anyway, this early patch helps prototyping this feature.

Note: The test case cannot unfortunately be reduced and is probably fragile.
llvm-svn: 242585