R600: Factorise VTX_WORD0 and VTX_WORD1 in tblgen def

Patch by: Vincent Lejeune

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
R600: Add a CONST_ADDRESS node to model constant buf read

Patch by: Vincent Lejeune

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
R600: rework handling of the constants

Remove Cxxx registers, add new special register - "ALU_CONST" and new
operand for each alu src - "sel". ALU_CONST is used to designate that the
new operand contains the value to override src.sel, src.kc_bank, src.chan
for constants in the driver.

Patch by: Vadim Girlin

Vincent Lejeune:
  - Use pointers for constants
  - Fold CONST_ADDRESS when possible

Tom Stellard:
  - Give CONSTANT_BUFFER_0 its own address space
  - Use integer types for constant loads

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>