[LICM] Hoist calls to readonly argmemonly functions even with stores in the loop

We know that an argmemonly function can only access memory pointed to by it's pointer arguments. Rather than needing to consider all possible stores as aliasing (as we do for a readonly function), we can only consider the aliasing of the pointer arguments.

Note that this change only addresses hoisting. I'm thinking about how to address speculation safety as well, but that will be a different change.

FYI, argmemonly disallows accessing memory through non-pointer typed arguments.  

Differential Revision: http://reviews.llvm.org/D12771

llvm-svn: 248220