nir/lower_io: fix handling for compact arrays with indirect derefs

this logic relies on constant indexing for compact arrays, but this is
frequently not the case for compact array builtins (e.g., gl_TessLevelOuter).
the usual strategy of lowering to temps isn't viable in TCS, which means
io lowering has to be able to handle indirect access to these builtins
without crashing

cc: mesa-stable

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27534>

src/compiler/nir/nir_lower_io.c

@@ -216,7 +216,7 @@ get_io_offset(nir_builder *b, nir_deref_instr *deref,
       p++;
    }
 
-   if (path.path[0]->var->data.compact) {
+   if (path.path[0]->var->data.compact && nir_src_is_const((*p)->arr.index)) {
       assert((*p)->deref_type == nir_deref_type_array);
       assert(glsl_type_is_scalar((*p)->type));