Revert r162713: "Add ATOMIC_LDR* pseudo-instructions to model atomic_load on ARM."

Revert r162713: "Add ATOMIC_LDR* pseudo-instructions to model atomic_load on ARM."

This wasn't the right way to enforce ordering of atomics.

We are already setting the isVolatile bit on memory operands of atomic
operations which is good enough to enforce the correct ordering.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162732 91177308-0d34-0410-b5e6-96231b3b80d8