message: Add a fast check in IsFusible for while and conditionals.
  Otherwise this check may take too long as it goes through all instructions in the computation.
  (Someday we may reenable this and I look forward to it).
diff: @@ -2588,6 +2588,9 @@ bool HloInstruction::IsFusible() const {
  switch (opcode_) {
  case HloOpcode::kDomain:
  case HloOpcode::kParameter:
  + case HloOpcode::kWhile:
  + case HloOpcode::kConditional:
  + case HloOpcode::kCall:
  return false;
  // Side effecting instrutions cannot be fused.
  default:
  