Set the loop unroll's threshold to 1024.
Set the loop unroll's threshold to 1024.

LLVM's default threshold is 150, too small. In general, the loop unroll
in GEN can improve the perfomance, so enlarge it to 1024.

Signed-off-by: Yang Rong <78759e37dc7114caed18f74e7515c5cb519dbf6d@intel.com>
Reviewed-by: "Song, Ruiling" <286a174c1339c6eff91abc41c3a1012c95030c95@intel.com>

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