Remove some more uses of subgraphs and more cleanup of the graph builder.

Remove some more uses of subgraphs and more cleanup of the graph builder.

Do not use subgraphs to implement the translation of simple branching
control flow, for the function body entry, or for labeled blocks.

Combine all the loop construction functions into a single one.

Resolve a possible problem with duplicate AST IDs used both for joined loop
break blocks, the normal loop exit, and for their common successor, by never
introducing the extra (successor) block and instead making the normal loop
exit a predecessor of the break join block.  There is a similar issue with
joined continue blocks.

Remove a (never needed) two-element zone list per each time we replace one
hydrogen value with another.

Review URL: http://codereview.chromium.org/6604002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7027 ce2b1a6d-e550-0410-aec6-3dcde31c8c00