nir/opt_varyings: simplify nir_io_semantics::num_slots of directly-indexed slots

Compaction only moves directly-indexed slots. This prevents unnecessary
num_slots > 1 from appearing in random slots.

Fixes: c66967b5cb - nir: add nir_opt_varyings, new pass optimizing and compacting varyings

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28431>

src/compiler/nir/nir_opt_varyings.c

@@ -3547,6 +3547,9 @@ relocate_slot(struct linkage_info *linkage, struct scalar_slot *slot,
          sem.location = new_semantic;
          sem.high_16bits = new_high_16bits;
 
+         /* This is never indirectly indexed. Simplify num_slots. */
+         sem.num_slots = 1;
+
          nir_intrinsic_set_io_semantics(intr, sem);
          nir_intrinsic_set_component(intr, new_component);