ir3: support texture and sampler index with offsets

"If this texture instruction has a nir_tex_src_texture_offset source, then the texture index is given by texture_index + texture_offset."

This fixes the failures for:

spec@arb_arrays_of_arrays@execution@sampler@fs-nested-struct-arrays-nonconst-nested-array

spec@arb_gl_spirv@execution@uniform@sampler2d-nonconst-nested-array

Signed-off-by: Amber Amber amber@igalia.com