i965/vec4: Allow propagation of instructions with saturate flag to sel

i965/vec4: Allow propagation of instructions with saturate flag to sel

When sel conditon is bounded within 0 and 1.0. This allows code as:
        mov.sat a b
        sel.ge  dst a 0.25F

To be propagated as:
        sel.ge.sat dst b 0.25F

v3: - Syntax clarifications in inst->saturate assignment
    - Remove extra parenthesis when assigning src_reg value
      from copy_entry (Matt Turner)
v4: - Take channels into consideration when propagating saturated instructions.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>