Replace GCC-specific intrinsic with portable alternative.
Not all supported compilers have GCC intrinsics, so this patch
uses the correct portable alternative.
Additionally, this patch fixes an off-by-one error. __builtin_ffs
returns the 1-based index of the least-significant 1-bit, but the
function expects the base 2 logarithm of the number, which is
equivalent to the 0-based index of the least-significant 1-bit.
Reviewed by: Keno Fischer
Differential Revision: http://reviews.llvm.org/D4284
llvm-svn: 211669