There is a federal law, 18 USC 4, against "misprision of felony", which states
Whoever, having knowledge of the actual commission of a felony
cognizable by a court of the United States, conceals and does not as
soon as possible make known the same to some judge or other person in
civil or military authority under the United States, shall be fined
under this title or imprisoned not more than three years, or both.
Various (but not all) violations of immigration laws count as felonies in being punishable with 2-5 years of prison (18 USC 1325).
In US v. Baumgartner, citing US. v. Goldberg, 862 F.2d 10 which cites the same conclusion from prior case law, says that the elements of misprision of felony are
(1) the principal committed and completed the felony alleged; (2) the
defendant had knowledge of the fact; (3) the defendant failed to
notify the authorities; and (4) the defendant took affirmative steps
to conceal the crime of the principal.
The first 3 elements are present, so whether or not the officer would be himself committing a felony hinges on the last element. (r.e. element 2, the law does not require defendant to have personally observed the commission of the felony, see the Baumgartner case). As the courts also said,
Mere knowledge of the commission of the felony or failure to report
the felony, standing alone, is insufficient to support a conviction
for a misprision of a felony.
It thus matters whether the officer engaged in action to affirmatively conceal the felony. An example of active concealment would be modifying a police report to remove information reporting such; or instructing a colleague to not include such information. However, reporting the crime but not forwarding the report to ICE would probably not be held to be active concealment, unless law enforcement officers have a general duty to report all felonies to competent authorities (an obligation which I don't think exists).