Workaround a LLVM/Clang 3.3 bug.

The LLVM/Clang 3.3 has a bug when compile a cl kernel with
assignment of a scalar to a vector data type. This patch
could work around this bug.

Signed-off-by: Zhigang Gong <zhigang.gong@intel.com>
Merge pull request #2318 from gongzg:2.4