CPU: Simplify the implementation of the decode cache.
CPU: Simplify the implementation of the decode cache.

Also reorganize it to make it more amenable to being rearranged later.

pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,hoangt/tpzsimul.gem5,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,hoangt/tpzsimul.gem5,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,vovojh/gem5,hoangt/tpzsimul.gem5,vovojh/gem5,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,vovojh/gem5,vovojh/gem5,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-