freedreno/ir3: lower immeds to const

freedreno/ir3: lower immeds to const

Helps reduce register pressure and instruction counts for immediates
that would otherwise require a mov into gpr.

total instructions in shared programs:          4455332 -> 4369297 (-1.93%)
total dwords in shared programs:                8807872 -> 8614432 (-2.20%)
total full registers used in shared programs:   263062 -> 250846 (-4.64%)
total half registers used in shader programs:   9845 -> 9845 (0.00%)
total const registers used in shared programs:  1029735 -> 1466993 (42.46%)

                 half       full      const      instr     dwords
    helped           0       10415           0       17861        5912
      hurt           0        1157       21458         947          33

Signed-off-by: Rob Clark <robclark@freedesktop.org>