ir3: Reset num when creating parallel copies

It may have been overwritten when folding in constants.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22075>

src/freedreno/ir3/ir3_merge_regs.c

@@ -472,6 +472,7 @@ create_parallel_copy(struct ir3_block *block)
          phi->srcs[pred_idx]->def = pcopy->dsts[j];
          pcopy->dsts[j]->flags |= phi->dsts[0]->flags & IR3_REG_SHARED;
          phi->srcs[pred_idx]->flags = pcopy->dsts[j]->flags;
+         phi->srcs[pred_idx]->num = INVALID_REG;
          j++;
       }
       assert(j == phi_count);