Improve how tf_xla_cluster_exclude_ops flag is processed.
Looks like we have 2 minor issues with how tf_xla_cluster_exclude_ops is processed in mark_for_compilation_pass:

1. cluster_exclude_op_list is repeatedly created and validated within a loop, which is inefficient;
2. GetOrCreateClusterExcludeList can be renamed to CreateClusterExcludeList, since there is no caching mechanism and a brand new list is created every time calling that function;

This PR targets at fixing these 2 issues.