Don't follow region-breaking instructions in WholeCFG

Summary: Including control flow arcs created by bytecodes like Switch isn't
supported, so let's ban those in WholeCFG like we do in HotTrace.

Reviewed By: ottoni, alexanderm

Differential Revision: D1421832
Don't include the initial block twice

Summary: We weren't adding the initial block to the visited set during the CFG
traversal, so we could end up adding it to the region twice, which caused
various problems.

Reviewed By: @ottoni

Differential Revision: D1421815