Disambiguate conversion cast for GCC

GCC 9 has problems with this.

mlir/include/mlir/IR/OperationSupport.h: In member function ‘mlir::Value mlir::MutableOperandRange::operator[](unsigned int) const’:
mlir/include/mlir/IR/OperationSupport.h:912:43: error: call of overloaded ‘OperandRange(const mlir::MutableOperandRange&)’ is ambiguous
  912 |     return static_cast<OperandRange>(*this)[index];
      |
mlir/include/mlir/IR/OperationSupport.h:789:21: note: candidate: mlir::OperandRange::OperandRange(const llvm::iterator_range<llvm::detail::indexed_accessor_
range_base<mlir::OperandRange, mlir::OpOperand*, mlir::Value, mlir::Value, mlir::Value>::iterator>&)
   using RangeBaseT::RangeBaseT;
                     ^~~~~~~~~~
mlir/include/mlir/IR/OperationSupport.h:786:7: note: candidate: constexpr mlir::OperandRange::OperandRange(const mlir::OperandRange&)
 class OperandRange final : public llvm::detail::indexed_accessor_range_base<
       ^~~~~~~~~~~~
mlir/include/mlir/IR/OperationSupport.h:786:7: note: candidate: constexpr mlir::OperandRange::OperandRange(mlir::OperandRange&&)