[dfsan] Introduce further optimization to reduce the number of union queries.

Specifically, do not compute a union if it is statically known that one
shadow set subsumes the other.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213100 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed