OpenCL, vector operations: Removed generator usage.

Current generator for vector operations turned out to be too limiting
for some hardware, so we are pulling back here. In particular:
 - no support for char and short
 - no support for hardware with workgroup size 1
 - number of kernel launches sometimes unnecessarily high

This is only the first step in a serious of follow-up commits for
other BLAS level 1 and 2 operations.