[LoopIdiom] Merge TBAA of adjacent stores when creating memset

Factor in the TBAA of adjacent stores instead of just the head store
when merging stores into a memset. We were seeing GVN remove a load that
had a TBAA that matched the 2nd store because GVN determined it didn't
match the TBAA of the memset. The memset had the TBAA of only the first
store.

i.e. Loading the field pi_ of shared_count after memset to create an
array of shared_ptr

template<class T>
class shared_ptr {
  T *p;
  shared_count refcount;
};

class shared_count {
  sp_counted_base *pi_;
};

Differential Revision: https://reviews.llvm.org/D122205