[OpenCL] Add attribute 'pure' to read_image built-in functions to enable optimizations.

Reviewers: Anastasia, yaxunl

Subscribers: pekka.jaaskelainen, pxli168, cfe-commits

Differential Revision: http://reviews.llvm.org/D21795

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@274122 91177308-0d34-0410-b5e6-96231b3b80d8