[OpenCL] Fix pipe builtin bugs

Summary:
1. Diag should be output if types are not the same.
2. Should compare using canonical type.
3. Refine the diag to be more clear.

Reviewers: yaxunl, Anastasia

Subscribers: MatsPetersson, pekka.jaaskelainen, cfe-commits

Differential Revision: http://reviews.llvm.org/D17955

llvm-svn: 264825