freedreno/ir3: optimize immed 2nd src to mad

freedreno/ir3: optimize immed 2nd src to mad

We can't encode immed sources for cat3 (mad) instructions, but we can
use const in first or third src.  We handled this case already, but we
weren't considering that we could lower immed to const.

For manhattan:

  total instructions in shared programs: 35202 -> 34718 (-1.37%)
  instructions in affected programs: 14931 -> 14447 (-3.24%)
  helped: 90
  HURT: 0
  total full in shared programs: 2451 -> 2359 (-3.75%)
  full in affected programs: 653 -> 561 (-14.09%)
  helped: 69
  HURT: 2

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>