freedreno/ir3: Lower output precision

freedreno/ir3: Lower output precision

This lowers mediump FS outputs to fp16 in the ir3 backend. For now
this is a modest improvement, which mostly helps us whittle down the
full mediump work.  Once the GLSL level support lands, then right hand
side of the store output intrinsics will be fp16 expressions and we'll
cancel out the fp16 -> fp32 -> fp 16 round trip here.

We've had different attempts at implementing this: rewriting stores in
the GLSL IR, lowering GLSL IR outputs to temporaries and inserting
conversions when writing the temporaries to the outputs.  In the end,
GLSL ends up getting in the way a lot and doing it at the nir level is
easier and still possible since we have the output var precisions.

This part of the fp16 work is more of a step on the way towards full
fp16 support and will add a few extra conversion instructions:

total instructions in shared programs: 8151 -> 8163 (0.15%)
instructions in affected programs: 1187 -> 1199 (1.01%)
helped: 4
HURT: 10

total nops in shared programs: 3146 -> 3152 (0.19%)
nops in affected programs: 563 -> 569 (1.07%)
helped: 5
HURT: 10

total non-nops in shared programs: 5005 -> 5011 (0.12%)
non-nops in affected programs: 92 -> 98 (6.52%)
helped: 0
HURT: 3

total dwords in shared programs: 12832 -> 12800 (-0.25%)
dwords in affected programs: 96 -> 64 (-33.33%)
helped: 1
HURT: 0

total last-baryf in shared programs: 118 -> 115 (-2.54%)
last-baryf in affected programs: 21 -> 18 (-14.29%)
helped: 1
HURT: 0

total full in shared programs: 424 -> 417 (-1.65%)
full in affected programs: 15 -> 8 (-46.67%)
helped: 7

HURT: 0
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3822>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3822>