message: Restore SingleBlockImplicitTerminator verification to mhlo.while
  The internal users have been cleaned up, so we can roll this forward again.
diff: @@ -578,7 +578,8 @@ def HLO_CaseOp: HLO_Op<"case", [
  def HLO_WhileOp: HLO_Op<"while", [
  RecursiveSideEffects,
  - SameOperandsAndResultType
  + SameOperandsAndResultType,
  + SingleBlockImplicitTerminator<"ReturnOp">
  ]>, BASE_HLO_WhileOp {
  let arguments = (ins HLO_TensorOrTuple:$val);
  