Add graph coloring register allocator.

Add graph coloring register allocator.

Refactored linear allocator into separate liveness annotation and allocation stages.
Added graph coloring allocator, use -fregs-graph to enable.
  New dump flags are
    -ddump-asm-native          -- output of cmm -> native transform.
    -ddump-asm-liveness        -- code annotated with register liveness info
    -ddump-asm-coalesce        -- output of register move coalescing
                                    (this is a separate pass when using the coloring allocator)
                                    (this could change in the future)
    -ddump-asm-regalloc        -- code after register allocation
    -ddump-asm-regalloc-stages -- blocks after each build/spill stage of coloring allocator
    -ddump-asm-conflicts       -- a global register liveness graph in graphviz format

The new register allocator will allocate some registers, but it's not
quite ready for prime-time yet. The spill code generator needs some work...