[AArch64] Fix LowerMGATHER to return the chain result for floating point gathers.

[craig.topper]

Summary

Found by adding asserts to LegalizeDAG to make sure custom legalized
results had the right types.

[kmclaughlin]

Thanks for this fix, LGTM!

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

SDValue AArch64TargetLowering::LowerMGATHER(SDValue Op,
   if (ResNeedsSignExtend)
     Opcode = getSignExtendedGatherOpcode(Opcode);
 
   SDValue Ops[] = {Chain, Mask, BasePtr, Index, InputVT, PassThru};
   SDValue Gather = DAG.getNode(Opcode, DL, VTs, Ops);
 
   if (VT.isFloatingPoint()) {
     SDValue Cast = getSVESafeBitCast(VT, Gather, DAG);
-    return DAG.getMergeValues({Cast, Gather}, DL);
+    return DAG.getMergeValues({Cast, Gather.getValue(1)}, DL);
   }
 
   return Gather;
 }
 
 SDValue AArch64TargetLowering::LowerMSCATTER(SDValue Op,
                                              SelectionDAG &DAG) const {
   SDLoc DL(Op);