panvk: add note about pan_lower_store_component requirements

This tripped me up in the multiview implementation. The commit message
that introduced the pass mentioned that we're relying on
nir_lower_io_to_temporaries, but this was dropped when it was copied to
the comment block.

Signed-off-by: Benjamin Lee <benjamin.lee@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31704>

src/panfrost/util/pan_lower_store_component.c

@@ -32,6 +32,9 @@
  * location_frac, we'll need to lower to a single varying store that collects
  * all of the channels together. This is because the varying instruction on
  * Midgard and Bifrost is slot-based, writing out an entire vec4 slot at a time.
+ *
+ * NOTE: this expects all stores to be outside of control flow, and with
+ * constant offsets. It should be run after nir_lower_io_to_temporaries.
  */
 static bool
 lower_store_component(nir_builder *b, nir_intrinsic_instr *intr, void *data)