message: Document SelectOp class
  This was missing from the commit that introduced SelectOp although the
  documentation was present in the LangRef.md.
diff: @@ -639,6 +639,17 @@ private:
  explicit MulIOp(const Operation *state) : BinaryOp(state) {}
  };
  +/// The "select" operation chooses one value based on a binary condition
  +/// supplied as its first operand. If the value of the first operand is 1, the
  +/// second operand is chosen, otherwise the third operand is chosen. The second
  +/// and the third operand must have the same type. The operation applies
  +/// elementwise to vectors and tensors. The shape of all arguments must be
  +/// identical. For example, the maximum operation is obtained by combining
  +/// "select" with "cmpi" as follows.
  +///
  +/// %2 = cmpi "gt" %0, %1 : i32 // %2 is i1
  +/// %3 = select %2, %0, %1 : i32
  +///
  class SelectOp : public Op<SelectOp, OpTrait::NOperands<3>::Impl,
  OpTrait::OneResult, OpTrait::HasNoSideEffect> {
  public:
  