R600: initial scheduler code

This is a skeleton for a pre-RA MachineInstr scheduler strategy. Currently
it only tries to expose more parallelism for ALU instructions (this also
makes the distribution of GPR channels more uniform and increases the
chances of ALU instructions to be packed together in a single VLIW group).
Also it tries to reduce clause switching by grouping instruction of the
same kind (ALU/FETCH/CF) together.

Vincent Lejeune:
 - Support for VLIW4 Slot assignement
 - Recomputation of ScheduleDAG to get more parallelism opportunities

Tom Stellard:
 - Fix assertion failure when trying to determine an instruction's slot
   based on its destination register's class
 - Fix some compiler warnings

Vincent Lejeune: [v2]
 - Remove recomputation of ScheduleDAG (will be provided in a later patch)
 - Improve estimation of an ALU clause size so that heuristic does not emit cf
 instructions at the wrong position.
 - Make schedule heuristic smarter using SUnit Depth
 - Take constant read limitations into account

Vincent Lejeune: [v3]
 - Fix some uninitialized values in ConstPair
 - Add asserts to ensure an ALU slot is always populated
Hexagon: Add encoding bits to the TFR64 instructions.
Set imMoveImm, isAsCheapAsAMove flags for TFRI instructions.