intel/fs: Assert when brw_fs_nir sees a nir_deref_instr

Since 09f1de97a7 "anv,i965: Lower away image derefs in the driver"
the backend compiler is not expected to handle any derefs, so let's
assert on it.

This helps identifying problems when a deref is not lowered and
"leaks" into the backend compiler.

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

src/intel/compiler/brw_fs_nir.cpp

@@ -454,7 +454,7 @@ fs_visitor::nir_emit_instr(nir_instr *instr)
       break;
 
    case nir_instr_type_deref:
-      /* Derefs can exist for images but they do nothing */
+      unreachable("All derefs should've been lowered");
       break;
 
    case nir_instr_type_intrinsic: