[SROA] Fix nondeterminism exposed by Simon's r299221.

Use a SmallSetSetVector instead of a SmallPtrSet as iterating
over the latter is not stable ('<' relies on addresses).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301599 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed