Fixes assertion failures in the CTS since Karol's cleanup when NIR started
noticing that we were reading an invalid component.
Fixes: 5450f1c9fb ("v3d: prefer using nir_src_comp_as_int over nir_src_as_const_value")
This is only exposed on V3D 4.1+, because we didn't have the TMU write
operations for images on 3.3 (To do GLES 3.1 there, you have to lower it
to SSBO load/stores, which is a problem to solve later).
Even without any clever optimization on the unpack operations, this gives
us a useful value for the channels read field, which we can use to avoid
ldtmu instructions to the no-op register.
instructions in affected programs: 890712 -> 881974 (-0.98%)
This is what the GLSL ES 310 spec tells us to do, but apparently the
"gather mode" flag doesn't imply it in the HW. Fixes
dEQP-GLES31.functional.texture.gather.basic.2d.rgba8.filter_mode.min_nearest_mipmap_linear_mag_linear
These have all been floating in my head, and while I've thought about
encoding them in issues on gitlab once they're enabled, they also make
sense to just have in the area of the code you'll need to work in.
Fixes simulator assertion failures in
dEQP-GLES3.functional.shaders.texture_functions.texture.samplercubeshadow_bias_fragment
and similar complicated cases.
