aco/value_numbering: clear hashmap between disconnected CFGs

There is no dominance-relationship between two disconnected CFGs,
thus no CSE is possible.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20853>

src/amd/compiler/aco_opt_value_numbering.cpp

@@ -489,6 +489,9 @@ value_numbering(Program* program)
          loop_headers.pop_back();
       }
 
+      if (block.logical_idom == (int)block.index)
+         ctx.expr_values.clear();
+
       if (block.logical_idom != -1)
          process_block(ctx, block);
       else