popcount: Provide function implementation rather than intrinsic redirect

amdgcn will need to override this

Reviewer: Aaron Watry <awatry@gmail.com>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 327042
GitOrigin-RevId: 17e8679493734b07aa764a9ec7cf040da35388e0
7 files changed