[Bug tree-optimization/123330] Optimization fails with standard branchless idiom with power of 2s

[tobi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123330

--- Comment #7 from Tobias Schlüter <tobi at gcc dot gnu.org> ---
ps sorry, forget to add the 64 in the count_zeros_branchless(), so please
ignore that function, as it is not equivalent.  The equivalent function is of
course the subject of this PR.

Note to self: no last-minute additions!