nir/deref: Don't try to compare derefs containing casts

nir/deref: Don't try to compare derefs containing casts

One day, we may want copy_prop_vars or other passes to be able to see
through certain types of casts such as when someone casts a uint64_t to
a uvec2.  However, for now we should just avoid casts all together.

Fixes: d8e3edb784d3a "nir/deref: Support casts and ptr_as_array in..."
Tested-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6072>