Spill metric

A spill metric is a heuristic metric used by register allocators to decide which registers to spill. Popular spill metrics are:
 * cost / degree - introduced in Chaitin's algorithm
 * cost / degree2 - emphasizes the spill's effect on neighbours
 * cost - emphasizes run time
 * minimising number of spill operations

Where cost is the estimated cost of spilling a value from registers into memory.