i965/vec4: implement d2b

i965/vec4: implement d2b

v2 (Curro):
  - Generate the flag register with a MOV with conditional_mod instead of a CMP
    instruction, which has the benefit that we can skip loading a DF
    0.0 constant.
  - Avoid the PICK_LOW_32BIT + MOV by using the flag result and a
    SEL to set the boolean result.

v3:
  - Fix comment (Matt)

Reviewed-by: Matt Turner <mattst88@gmail.com>