nir/tex_instr: Rename the indirect source type and add an array size

nir/tex_instr: Rename the indirect source type and add an array size

In particular, we rename nir_tex_src_sampler_index to _sampler_offset and
add a sampler_array_size field to nir_tex_instr.  This way we can pass the
size of sampler arrays through to backends even after removing the variable
information and, with it, the type.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>