[TargetLowering][RISCV] Propagate fastmath flags for the vector operations emitted in expandVecReduce. (#85164)

We used the fastmath flags for any scalar ops created, but not vector.

Author: topperc

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -10694,7 +10694,7 @@ SDValue TargetLowering::expandVecReduce(SDNode *Node, SelectionDAG &DAG) const {
 
       SDValue Lo, Hi;
       std::tie(Lo, Hi) = DAG.SplitVector(Op, dl);
-      Op = DAG.getNode(BaseOpcode, dl, HalfVT, Lo, Hi);
+      Op = DAG.getNode(BaseOpcode, dl, HalfVT, Lo, Hi, Node->getFlags());
       VT = HalfVT;
     }
   }