[X86] Replace more calls to getZeroVector with regular getConstant.

getZeroVector produces a specifically canonicalized zero vector, but we can just let DAG legalization take care of it.

The test changes are because MULH lowering happens later than it should and this change gave us the opportunity to constant fold away a multiply during a DAG combine before the build_vector got legalized with a bitcast.

llvm-svn: 347290