pown: Use denormal path only

It's OK to either flush to 0 or return denormal result if the device
does not support denormals. See sec 7.2 and 7.5.3 of OCL specs
Fixes CTS on carrizo and turks.
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewer: Aaron Watry <awatry@gmail.com>

llvm-svn: 330206