Split IntrReadArgMem into IntrReadMem and IntrArgMemOnly

Summary:
IntrReadWriteArgMem simply becomes IntrArgMemOnly.

So there are fewer intrinsic properties that express their orthogonality
better, and correspond more closely to the corresponding IR attributes.

Suggested by: Philip Reames

Reviewers: joker.eph, reames, tstellarAMD

Subscribers: jholewinski, arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D19291

llvm-svn: 267021