Polish atomic pointers

Summary:
I didn't realize that we already allowed atomic load/store of pointers,
it was added in 2012 by r162146. This patch updates the documentation
and tightens the verifier by using DataLayout to make sure that the
stored size is byte-sized and power-of-two. DataLayout is also used for
integers, and while I'm here I updated the corresponding code for
cmpxchg and rmw.

See the following discussion for context and upcoming changes to
add floating-point and vector atomics:
  https://groups.google.com/forum/#!topic/llvm-dev/Nh0P_E3CRoo/discussion

Reviewers: reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15512

llvm-svn: 255931