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

Is BLOCK_SIZE = 16 (as local_size_0)? In such case this local mem pattern is not good. There is recommendation to use pitch = 16+1 instead of 16 (s_train[j * 17 + lidx]). Is it possible to try this pattern?