nir: Removing uses of SSA defs destroys SSA liveness

The liveness information will be a superset of real liveness so it's
unlikely something will explode if it tries to use it.  However, it is
out-of-date and should be re-run if someone really wants it.

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12186>

src/compiler/nir/nir.h

@@ -3100,7 +3100,8 @@ typedef enum {
     *   - nir_block::live_out
     *
     * A pass can preserve this metadata type if it never adds or removes any
-    * SSA defs (most passes shouldn't preserve this metadata type).
+    * SSA defs or uses of SSA defs (most passes shouldn't preserve this
+    * metadata type).
     */
    nir_metadata_live_ssa_defs = 0x4,