Add ATOMIC_LDR* pseudo-instructions to model atomic_load on ARM.

Add ATOMIC_LDR* pseudo-instructions to model atomic_load on ARM.

It is not safe to use normal LDR instructions because they may be
reordered by the scheduler. The ATOMIC_LDR pseudos have a mayStore flag
that prevents reordering.

Atomic loads are also prevented from participating in rematerialization
and load folding.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162713 91177308-0d34-0410-b5e6-96231b3b80d8