Match failed instruction selection error

     Could you please tell me how can I solve the following problem in which I get a strange instruction selection error ("Match failed at index 0" - see below for details).
I have a shufflevector instruction that has as first operand a MaskedGatherSDNode whose index is (in LLVM IR):
%5 = sext <8 x i16> %varVecIndexInnerLoop to <8 x i64>, !dbg !24
%varVecIndexInnerLoop = phi <8 x i16> [ zeroinitializer, ], [ %vecIndexAddRes, %vector.body ]
So the index of the MaskedGatherSDNode is in another basic-block.

 My problem is that I try to reuse the index in another Machine SDNode subtract instruction \(because I want to use the same register as in the index\), but then I get the following instruction selection error:

      Entered ConnexDAGToDAGISel::Select(): Selecting Node = t45: v8i64 = sign_extend t44, test.c:39:5
      Opcode = 157
      ConnexDAGToDAGISel::Select(): default case: Opcode = 157
      ISEL: Starting pattern match
       Initial Opcode index to 0
       Match failed at index 0
      LLVM ERROR: Cannot select: t45: v8i64 = sign_extend t44, test.c:39:5
       t44: v8i16,ch = CopyFromReg t0, Register:v8i16 %6, test.c:39:5
       t43: v8i16 = Register %6
      In function: Stencil

     Can I avoid this error maybe by replicating the index of the MaskedGatherSDNode as a separate node?

   Thank you very much,