integer: Add popcount implementation using ctpop intrinsic

Also copy/modify the unary_intrin.inc from math/ to make the
intrinsic declaration somewhat reusable.

Passes CL CTS integer_ops/test_integer_ops popcount tests for CL 1.2

Tested-by on GCN 1.0 (Pitcairn)

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 312854