[X86] Fix missed selection of non-temporal store of zero vector.

When the input to a store instruction was a zero vector, the backend
always selected a normal vector store regardless of the non-temporal
hint. This is fixed by this patch.

This fixes PR19370.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220054 91177308-0d34-0410-b5e6-96231b3b80d8