Treat states after return, break, and continue as dead - #2556
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ThrudPrimrose
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acalotoiu
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DeadStateElimination walks the CFG from the start block and follows the edges out of ReturnBlock, BreakBlock and ContinueBlock, so whatever sits after one of them still counts as reachable. Control flow does not continue past those blocks, so those states are dead and should go.
The fix is to stop the traversal when it reaches one of the three. Anything reachable only through them then falls out of the visited set and gets removed the same way as any other unreachable block.
Tests cover a return at the top level, and a break and a continue inside a loop region. There is also a test where a state is reachable both through the return and around it, so the pass has to keep it. That one passes before the change as well, it is there to catch the traversal being cut too early.
Closes #2051.