related issues: #5423 #5476

connectedComponents and connectedComponentsWithStats in
imgproc accept 8-bit single channel image only for input.

See https://github.com/Itseez/opencv/issues/5721

COMMENTS:
* The second __syncthreads() is necessary, I am sure of that.
* The code works without the first __syncthreads() too, but I have however added it for symmetry. Anyway it doesn't affect time performances, I have checked it with some profiling with nvvp

In oder to guarantee SIMDness between the two compilation steps we choose the
highest SIMD size that is optimized in the kernel for the first step.

wsfixed