OCL: Optimize OpenCL version of BFMatcher

Using vectors instead scalars, if data was aligned.

Report:
http://ocl.itseez.com/intel/export/perf/pr/3256/report/

check_regression=*OCL*BruteForceMatcher*
test_filter=*OCL*BruteForceMatcher*
test_modules=features2d
build_examples=OFF

does this barrier really need here?