[x86] simplify getZeroVector() ; NFCI

Let DAG.getConstant() handle the splatting; there's no need
to repeat that logic here.

See also:
http://reviews.llvm.org/rL258833
http://reviews.llvm.org/rL260582



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@260609 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed