[Attributor] Reorganize AAHeapToStack

In order to simplify future extensions, e.g., the merge of
AAHeapToShared in to AAHeapToStack, we reorganize AAHeapToStack and the
state we keep for each malloc-like call. The result is also less
confusing as we only track malloc-like calls, not all calls. Further, we
only perform the updates necessary for a malloc-like to argue it can go
to the stack, e.g., we won't check all uses if we moved on to the
"must-be-freed" argument.

This patch also uses Attributor helps to simplify the allocated size,
alignment, and the potentially freed objects.

Overall, this is mostly a reorganization and only the use of the
optimistic helpers should change (=improve) the capabilities a bit.

Differential Revision: https://reviews.llvm.org/D104993