ocl_calcOpticalFlowPyrLK optimizations

1. decrease branch number in CL code by replacing them into weights
2. decrease local mem pressure in reduce operation by using private variables
3. decrease image sampler pressure by caching data into local memory
4. remove unnecessary sync point on the HOST side.
Merge remote-tracking branch 'github/master' into pullreq/140319-PyrLKOpticalFlow

Conflicts:
	modules/video/src/opencl/pyrlk.cl
fix previous merge error
Merge pull request #2566 from krodyush:pullreq/140319-PyrLKOpticalFlow