Avoid unnecessary SWAP in cryptsha512-opencl.
- 8% gain on GTX 570 (now it runs at 14.2K c/s)
- Also, on HD 6770 (driver 14.6beta) achieve the original performance (from 12.6 driver).
Merge pull request #740 from claudioandre/bleeding-jumbo

Avoid unnecessary SWAP in cryptsha512-opencl.