[LegalizeDAG] Share Vector/Scalar CTTZ Expansion

As suggested on D53258, this patch demonstrates sharing common CTTZ expansion code between VectorLegalizer and SelectionDAGLegalize by putting it in TargetLowering.

I intend to move CTLZ and (scalar) CTPOP over as well and then update D53258 accordingly.

Differential Revision: https://reviews.llvm.org/D53474

llvm-svn: 345039