[MemorySSA] Allow reordering of loads that alias in the presence of volatile loads.

[MemorySSA] Allow reordering of loads that alias in the presence of volatile loads.

Summary:
Make MemorySSA allow reordering of two loads that may alias, when one is volatile.
This makes MemorySSA less conservative and behaving the same as the AliasSetTracker.
For more context, see D16875.

LLVM language reference: "The optimizers must not change the number of volatile operations or change their order of execution relative to other volatile operations. The optimizers may change the order of volatile operations relative to non-volatile operations. This is not Java’s “volatile” and has no cross-thread synchronization behavior."

Reviewers: george.burgess.iv, dberlin

Subscribers: sanjoy, reames, hfinkel, llvm-commits, Prazek

Differential Revision: https://reviews.llvm.org/D41525

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321382 91177308-0d34-0410-b5e6-96231b3b80d8