Substantially speed up combiner-aa in the following ways:

1. Switch from an std::set to a SmallPtrSet for visited chain nodes.
2. Do not force the recursive flattening of token factor nodes, regardless of
   use count.
3. Immediately process newly created TokenFactor nodes.

Also, improve combiner-aa by teaching it that loads to non-overlapping offsets
of relatively aligned objects cannot alias.

These changes result in a >5x speedup for combiner-aa on most testcases.

llvm-svn: 81816