[LVI] Avoid iterator invalidation in LazyValueInfoCache::threadEdge

Do that by copying out the elements to another SmallPtrSet.
Follow up from r245309.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245590 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed