nir: Fix uninitialized use of 'replacement'.

For intrinsics we don't care about, just skip to the next loop iteration
and process the next instruction.  We don't want to execute the rest of
the code.

This was a bug in commit cdfc05ea6e.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/compiler/nir/nir_lower_io.c

@@ -369,7 +369,7 @@ nir_lower_io_block(nir_block *block,
          break;
 
       default:
-         break;
+         continue;
       }
 
       if (nir_intrinsic_infos[intrin->intrinsic].has_dest) {