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/compiler/nir/nir.h

@@ -3329,6 +3329,8 @@ bool nir_opt_algebraic_before_ffma(nir_shader *shader);
 bool nir_opt_algebraic_late(nir_shader *shader);
 bool nir_opt_constant_folding(nir_shader *shader);
 
+bool nir_opt_combine_stores(nir_shader *shader, nir_variable_mode modes);
+
 bool nir_opt_global_to_local(nir_shader *shader);
 
 bool nir_copy_prop(nir_shader *shader);