v3d: use inc/dec tmu operation with atomic sub/add of 1

v3d: use inc/dec tmu operation with atomic sub/add of 1

Among other things, this avoid the need of loading 1/-1 constants (so
one less operation).

The removed comment suggest the option of adding support on NIR for
inc/dec. Intel just uses an auxiliar method to get which hw operation
is needed, so no lowering is needed. And at the same time, being so
small, seems unreasonable to try to add a general one on NIR
itself. It is more easy to just adapt the method here (that is what
the patch does right now).

It is worth to note that we are not getting any change on shader-db
stats because all those methods are used on the usual shader-db set
with shaders needing GLSL > 4.2. In general there aren't too many GLSL
ES 3.1 tests.

As an alternative, we captured the GLES3/GLSL31/GLS32 used on
vk-gl-cts, even if that is not a real life usage of shaders. With
those we get the following:

total instructions in shared programs: 1217022 -> 1217013 (<.01%)
instructions in affected programs: 117 -> 108 (-7.69%)
helped: 6
HURT: 0
helped stats (abs) min: 1 max: 2 x̄: 1.50 x̃: 1
helped stats (rel) min: 3.57% max: 10.00% x̄: 8.09% x̃: 9.09%
95% mean confidence interval for instructions value: -2.07 -0.93
95% mean confidence interval for instructions %-change: -10.54% -5.64%
Instructions are helped.

Note that the shaders helped are really low because most of the
vk-gl-cts tests using AtomicInc/Dec/Add are mostly used on compute
shaders. Although right now there is a branch around with CS support,
the usual is doing the stats against master.

Reviewed-by: Eric Anholt <eric@anholt.net>