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

Yes. I changed a code of this PR, and last (perfomance report)[http://ocl.itseez.com/intel/export/perf/pr/3256/report/detail/20141017-144957-22ff1e8-3787388_SHARK-BAY-HD4600_master_win64-vc11_regression-vs-nightly_iGPU-HD4600.html?t0r3=0] show perf improvment.