freedreno/ir3: optimize less in glsl

freedreno/ir3: optimize less in glsl

Rely on nir for optimization, to reduce compile times.  Very minimal impact
on shader-db:

  total instructions in shared programs:          104170 -> 104199 (0.03%)
  total dwords in shared programs:                209664 -> 209728 (0.03%)
  total full registers used in shared programs:   7156 -> 7161 (0.07%)
  total half registers used in shader programs:   109 -> 109 (0.00%)
  total const registers used in shared programs:  24222 -> 24224 (0.01%)

                   half       full      const      instr     dwords
      helped          12         107         103         112          98
        hurt          11         104         105         115         102

But shader db runtime dropped from ~29.3s user to ~20.4s user.

Signed-off-by: Rob Clark <robdclark@gmail.com>