Simplify EH control flow by observing that EH scopes form a simple

Simplify EH control flow by observing that EH scopes form a simple
hierarchy of delegation, and that EH selector values are meaningful
function-wide (good thing, too, or inlining wouldn't work).
2,3d
1a
hierarchy of delegation and that EH selector values have the same
meaning everywhere in the function instead of being meaningful only
in the context of a specific selector.

This removes the need for routing edges through EH cleanups,
since a cleanup simply always branches to its enclosing scope.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137293 91177308-0d34-0410-b5e6-96231b3b80d8