message: [spirv] Check that operand of `spirv::CompositeExtractOp` is constant while folding.
  Closes
  COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/281 from denis0x0D:sandbox/composite_ex_fold
diff: @@ -336,6 +336,9 @@ static void printVariableDecorations(Operation *op, OpAsmPrinter &printer,
  // `indices`. Returns a null Attribute if error happens.
  static Attribute extractCompositeElement(Attribute composite,
  ArrayRef<unsigned> indices) {
  + // Check that given composite is a constant.
  + if (!composite)
  + return {};
  // Return composite itself if we reach the end of the index chain.
  if (indices.empty())
  return composite;
  