[X86] Remove unnecessary setting of CTTZ_ZERO_UNDEF to Custom for vector types...

[X86] Remove unnecessary setting of CTTZ_ZERO_UNDEF to Custom for vector types where we can't do any better than the Custom lowering of CTTZ. LegalizeVectorOps will expand to CTTZ since its marked Custom.

CTTZ_ZERO_UNDEF can be custom lowered specially if CTLZ is supported. Otherwise CTTZ and CTTZ_ZERO_UNDEF are handled the same way by using CTPOP and bitmath.

llvm-svn: 266952