sw/vec_iree: Fix ukernel breakage due to upstream macro removal

The macro IREE_UK_ASSUME_UNREACHABLE was removed at upstream https://github.com/openxla/iree/pull/16261

Change-Id: I6a1615ff45dc8cac70f0515e82bcdc6e5e98c904

ukernel/mmt4d_tile.c

diff --git a/ukernel/mmt4d_tile.c b/ukernel/mmt4d_tile.c
index a45df27..93c5ee6 100644
--- a/ukernel/mmt4d_tile.c
+++ b/ukernel/mmt4d_tile.c
@@ -525,7 +525,7 @@
     case iree_uk_mmt4d_type_f32f32f32:
       return iree_uk_mmt4d_tile_f32f32f32_generic;
     default:
-      IREE_UK_ASSUME_UNREACHABLE;
+      // Shouldn't happen, validated earlier.
       return 0;
   }
 }