[X86][BF16] Add test coverage for AVX-NE-CONVERT

Split from D158952.

Author: phoebewang

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -8479,7 +8479,8 @@ X86TargetLowering::LowerBUILD_VECTOR(SDValue Op, SelectionDAG &DAG) const {
   if (VT.getVectorElementType() == MVT::i1 && Subtarget.hasAVX512())
     return LowerBUILD_VECTORvXi1(Op, DAG, Subtarget);
 
-  if (VT.getVectorElementType() == MVT::bf16 && Subtarget.hasBF16())
+  if (VT.getVectorElementType() == MVT::bf16 &&
+      (Subtarget.hasAVXNECONVERT() || Subtarget.hasBF16()))
     return LowerBUILD_VECTORvXbf16(Op, DAG, Subtarget);
 
   if (SDValue VectorConstant = materializeVectorConstant(Op, DAG, Subtarget))