[PM/AA] Fix *numerous* serious bugs in GlobalsModRef found by

[PM/AA] Fix *numerous* serious bugs in GlobalsModRef found by
inspection.

While we want to handle calls specially in this code because they should
have been modeled by the call graph analysis that precedes it, we should
*not* be re-implementing the predicates for whether an instruction reads
or writes memory. Those are well defined already. Notably, at least the
following issues seem to be clearly missed before:
- Ordered atomic loads can "write" to memory by causing writes from other
  threads to become visible. Similarly for ordered atomic stores.
- AtomicRMW instructions quite obviously both read and write to memory.
- AtomicCmpXchg instructions also read and write to memory.
- Fences read and write to memory.
- Invokes of intrinsics or memory allocation functions.

I don't have any test cases, and I suspect this has never really come up
in the real world. But there is no reason why it wouldn't, and it makes
the code simpler to do this the right way.

While here, I've tried to make the loops significantly simpler as well
and added helpful comments as to what is going on.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242281 91177308-0d34-0410-b5e6-96231b3b80d8