backend: refine hypot function
backend: refine hypot function

the test OCL_Magnitude of opencv is slow on beignet because
of hypot. refine the hypot, change algorithm and remove
unnecessary code to get 30% up

Signed-off-by: rander.wang <5299db4a9bf784920a3e558099f6736bcd8939e6@intel.com>
Reviewed-by: Pan Xiuli <e5a665b1e626c5dc0826076a8b5a209eb48bf6ee@intel.com>

wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet