[OpenCL] Refine pipe builtin support

Summary:
Refine the type builtin support as the request with
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20160201/148637.html

Reviewers: pekka.jaaskelainen, Anastasia, yaxunl

Subscribers: rsmith, cfe-commits

Differential Revision: http://reviews.llvm.org/D16876

llvm-svn: 262692