Add iterative coalescing to graph coloring allocator
Iterative coalescing interleaves conservative coalesing with the regular simplify/scan passes. This increases the chance that nodes will be coalesced as they will have a lower degree than at the beginning of simplify. The end result is that more register to register moves will be eliminated in the output code, though the iterative nature of the algorithm makes it slower compared to non-iterative coloring. Use -fregs-iterative for graph coloring allocation with iterative coalescing -fregs-graph for non-iterative coalescing. The plan is for iterative coalescing to be enabled with -O2 and have a quicker, non-iterative algorithm otherwise. The time/benefit tradeoff between iterative and not is still being tuned - optimal graph coloring is NP-hard, afterall..