nir: Silence GCC maybe-uninitialized warnings.

nir/nir_control_flow.c: In function ‘split_block_cursor.isra.11’:
nir/nir_control_flow.c:460:15: warning: ‘after’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       *_after = after;
               ^
nir/nir_control_flow.c:458:16: warning: ‘before’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       *_before = before;
                ^

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>

src/glsl/nir/nir_control_flow.c

@@ -452,6 +452,9 @@ split_block_cursor(nir_cursor cursor,
          before = split_block_before_instr(nir_instr_next(cursor.instr));
       }
       break;
+
+   default:
+      unreachable("not reached");
    }
 
    if (_before)