[X86] In LowerVectorCTPOP use ISD::ZERO_EXTEND/ISD::TRUNCATE instead of the target specific nodes.

The target independent nodes will get legalized to the target specific nodes by their own legalization process. Someday I'd like to stop using a target specific for zero extends and truncates of legal types so the less places we reference the target specific opcode the better.

llvm-svn: 320863