R600: Implement zero undef variants of ctlz/cttz

v2: use ffbh/l if available
v3: Rebase on top of Matt's SI patches

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Tom Stellard <tom@stellard.net>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213072 91177308-0d34-0410-b5e6-96231b3b80d8