Add IntrWrite[Arg]Mem intrinsic property

Summary:
This property is used to mark an intrinsic that only writes to memory, but
neither reads from memory nor has other side effects.

An example where this is useful is the llvm.amdgcn.buffer.store.format.*
intrinsic, which corresponds to a store instruction that goes through a special
buffer descriptor rather than through a plain pointer.

With this property, the intrinsic should still be handled as having side
effects at the LLVM IR level, but machine scheduling can make smarter
decisions.

Reviewers: tstellarAMD, arsenm, joker.eph, reames

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D18291

llvm-svn: 266826