[X86][SSE] Improve CTTZ lowering when CTLZ is legal

If we have better CTLZ support than CTPOP, then use cttz(x) = width - ctlz(~x & (x - 1)) - and remove the CTTZ_ZERO_UNDEF handling as it no longer gives better codegen.

Similar to rL344447, this is also closer to LegalizeDAG's approach

llvm-svn: 344448