Add MachineMemOperand::isUnordered().

This means the same as LoadInst/StoreInst::isUnordered(), and implies
!isVolatile().

Atomic loads and stored are also ordered, and this is the right method
to check if it is safe to reorder memory operations. Ordered atomics
can't be reordered wrt normal loads and stores, which is a stronger
constraint than volatile.

llvm-svn: 162859