Refactor construction of switch statements so it doesn't use class
HSubgraph.

There are also a few improvements.  We do not use an auxiliary list of
comparisons because they're embedded as a linked list in the graph
under construction.  We share a common break block for all breaks from
the same switch.  We do not insert empty blocks unless necessary to
maintain edge-split form.

There is also a bug fix.  The entry to a clause body is a potential
join and must have a join ID set, otherwise deoptimization within the
body can go to an unpredictable place in the unoptimized code.

Review URL: http://codereview.chromium.org/6650021

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7099 ce2b1a6d-e550-0410-aec6-3dcde31c8c00