nir/vars_to_ssa: Properly ignore variables with complex derefs

nir/vars_to_ssa: Properly ignore variables with complex derefs

Because the core principle of the vars_to_ssa pass is that it globally
(within a function) looks at all of the uses of a never-indirected path
and does a full into-SSA on that path, it can't handle a path which has
any chance of having aliasing.  If a function_temp variable has a cast
or anything else which may cause aliasing, we have to assume that all
paths to that variable may alias and ignore the entire variable.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>