inject pre-finally and after-finally edges into flow graph to possibly ignore pre-finally during flow walk

[vladima]

fix proposal for #12205

I've tried the approach with copying the flow graph in finally block and decided not to use it since in presence of nested try-finally blocks in becomes quite complicated. As an alternative I'd like to propose different approach: for try-finally blocks we inject two extra edges in the flow graph. One edge will link pre-try flow with pre-finally label, another will act as a proxy for post-finally flow. If during the flow walk we touch the post-finally flow edge we can mark matching pre-finally edge as blocked so it will be ignored. The overhead of this approach is also much smaller than copying - max two objects per protected region which sounds like a reasonable thing.

// cc @ahejlsberg, @mhegazy