OpenCL: handle ternary operator when the condition is a vector

When the condition is a vector, OpenCL specifies additional
requirements on the operand types, and also the operations
required to determine the result type of the operator. This is a
combination of OpenCL v1.1 s6.3.i and s6.11.6, and the semantics
remain unchanged in later versions of OpenCL.

llvm-svn: 228118