Do conservative coalescing in register allocator

Do conservative coalescing in register allocator

Avoid coalescing nodes in the register conflict graph if the
new node will not be trivially colorable. Also remove the
front end aggressive coalescing pass.

For typical Haskell code the graph coloring allocator now does
about as well as the linear allocator.

For code with a large amount of register pressure it does much
better, but takes longer.

For SHA1.lhs from darcs on x86

          spills    reloads    reg-reg-moves
          inserted   inserted  left in code   compile-time
  linear    1068      1311        229            7.69(s)
  graph      387       902        340           16.12(s)