nir: support indirect indexing samplers in struct arrays

nir: support indirect indexing samplers in struct arrays

As a bonus we get indirect support for arrays of arrays for free.

V5: couple of small clean-ups suggested by Jason.

V4: fix struct member location caclulation, use nir_ssa_def rather than
nir_src for the indirect as suggested by Jason

V3: Use nir_instr_rewrite_src() with empty src rather then clearing
the use_link list directly for the old indirects as suggested by Jason

V2: Fixed validation error in debug build

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>