Merge def and gen sets

Merge def and gen sets

For live-variable analysis it does not matter if def includes
variables that are previously use in the same block, the data flow
equations still have the same result. As such there is no need to
compute separate gen & def sets.

I'm keeping the name "def", because use of "gen" in this context is
pretty confusing (gen is usually the use set, not the def set).