[BOLT] Consistent DFS ordering for landing pads

Summary:
The list of landing pads in BinaryBasicBlock was sorted by their address
in memory. As a result, the DFS order was not always deterministic.
The change is to store landing pads in the order they appear in invoke
instructions while keeping them unique.

Also, add Throwers verification to validateCFG().

(cherry picked from FBD6529032)