Avoid undefined behavior in the x86 bmi header file by explicitly checking for 0...

Avoid undefined behavior in the x86 bmi header file by explicitly checking for 0 before calling __builtin_ctz. Without this the optimizers may take advantage of the undefined behavior and produce incorrect results. LLVM itself still needs to be taught to merge the zero check into the llvm.cttz with defined zero behavior.

llvm-svn: 221065