T-API: optimization of cv::accumulate, cv::accumulateSquare, cv::accumulateProduct, cv::accumulateWeighted; added strategies in cv::predictOptimalVectorWidth

**Performance report:**
http://ocl.itseez.com/intel/export/perf/pr/2956/report/

check_regression=*OCL*
test_filter=*OCL*
test_modules=imgproc,core
build_examples=OFF
Don't use magic numbers. Use CV_8U/CV_8S/etc for indexes.