i965/fs: Allocate single register at a time for constants.

i965/fs: Allocate single register at a time for constants.

No instruction counts changed, but:

  total cycles in shared programs: 64834502 -> 64781530 (-0.08%)
  cycles in affected programs: 16331544 -> 16278572 (-0.32%)
  helped: 4757
  HURT: 4288

  GAINED: 66
  LOST:   20

I remember trying this when I first wrote the pass, but it wasn't
helpful at the time.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>