[X86] MatchVectorAllZeroTest - fix bug when splitting vectors of large elements

DAG::SplitVector only works with vectors with even numbers of elements, when splitting vectors with large (illegal) element widths, we are likely to split down to <1 x iXXX>.

In such cases, pre-bitcast to a <X x i64> type to ensure splitting will always succeed.

Thanks to @alexfh for identifying this.