New infrastructure for try/finally in iterators (still unused)

New infrastructure for try/finally in iterators (still unused)

* flowanalysis.cs (FlowBranching.AddResumePoint): New.
(FlowBranchingToplevel.AddResumePoint): Hook into
ToplevelBlock.AddResumePoint.
(FlowBranchingTryCatch): Move CS01626 and CS01631 checks here.
(FlowBranchingException): Hook into ExceptionBlock.AddResumePoint.
* statement.cs (ToplevelBlock.AddResumePoint): New.  Collect
resume points and assign program-counter values.
(ExceptionBlock.AddResumePoint): Collect resume points for
de-muxer at the top of try block.
* iterators.cs (Yield.CheckContext): Simplify.
(Yield.Resolve): Use FlowBranching.AddResumePoint.

svn path=/trunk/mcs/; revision=99881