T-API: optimized reduction operations

What was done:
* Simplified index calculation in case of continuous matices
* Vectorized some kernels (sum, countNonZero, UMat::dot) in case of single channel

Performance report can be viewed here: 
http://ocl.itseez.com/intel/export/perf/pr/2801/report/

check_regression=*OCL*Norm*:*OCL*MinMaxLoc*:*OCL*Dot*:*OCL*Sum*:*OCL*CountNonZeroFix*:*OCL*PSNR*
test_modules=core
test_filter=*OCL*Norm*:*OCL*MinMaxLoc*:*OCL*Dot*:*OCL*Sum*:*OCL*CountNonZeroFix*
build_examples=OFF