nir: Add a pass to combine store_derefs to same vector

v2: (all from Jason)
    Reuse existing function for the end of the block combinations.
    Check the SSA values are coming from the right place in tests.
    Document the case when the store to array_deref is reused.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/intel/compiler/brw_nir.c

@@ -555,6 +555,7 @@ brw_nir_optimize(nir_shader *nir, const struct brw_compiler *compiler,
       }
       OPT(nir_opt_copy_prop_vars);
       OPT(nir_opt_dead_write_vars);
+      OPT(nir_opt_combine_stores, nir_var_all);
 
       if (is_scalar) {
          OPT(nir_lower_alu_to_scalar);