There is no need to use a target-specific intrinsic to implement
_bit_scan_forward or _bit_scan_reverse, reimplementing them using
generic intrinsics makes it more likely that the middle end will
understand what's going on.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272564 91177308-0d34-0410-b5e6-96231b3b80d8