PR19957: [OpenCL] Incorrectly accepts implicit address space conversion with ternary operator.

Generates addrspacecast instead of bitcast for ternary operator when necessary, and diagnose ternary operator with incompatible second and third operands.

https://llvm.org/bugs/show_bug.cgi?id=19957

Differential Revision: http://reviews.llvm.org/D17412

llvm-svn: 266111