Avoid undefined behavior in the x86 lzcnt header file by explicitly checking for 0 before calling __builtin_clz. 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.ctlz with defined zero behavior.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221064 91177308-0d34-0410-b5e6-96231b3b80d8