T-API: minor optimization of cv::LUT

**Description:**
* optimized index calculation
* don't use vectors for `CV_32FC1` (used `ocl::predictOptimalVectorWidth`)

**Performance report:**
http://ocl.itseez.com/intel/export/perf/pr/2927/report/

check_regression=*OCL*LUT*
test_filter=*OCL*LUT*
test_modules=core
build_examples=OFF