ir3: call opt_deref in optimization loop to remove unnecessary tex casts
Otherwise we may be left with such casts:
vec1 32 ssa_72 = deref_var &shadow_map (uniform sampler2D)
vec1 32 ssa_73 = deref_cast (texture2D *)ssa_72 (uniform texture2D)
vec1 32 ssa_74 = deref_cast (sampler *)ssa_72 (uniform sampler)
vec1 32 ssa_76 = (float32)tex ssa_73 (texture_deref), ssa_74 (sampler_deref), ssa_75 (coord), ssa_64 (comparator)
And crash in ycbcr lowering since we aren't able to follow deref chain.
Fixes crash in GFXBench Aztec Ruins Vulkan tests. See issue: #5945 (closed)
Cc: mesa-stable