SmallPtrSet: Reuse DenseMapInfo's pointer hash function instead of inventing a bad one ourselves.

DenseMap's hash function uses slightly more entropy and reduces hash collisions
significantly.  I also experimented with Hashing.h, but it didn't gave a lot of
improvement while being much more expensive to compute.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154996 91177308-0d34-0410-b5e6-96231b3b80d8