// Absorb input zero extension into 'rotate then insert selected bits [into zero]'.
(RISBGZ (MOVWZreg x) {r}) && r.InMerge(0xffffffff) != nil => (RISBGZ x {*r.InMerge(0xffffffff)})
(RISBGZ (MOVHZreg x) {r}) && r.InMerge(0x0000ffff) != nil => (RISBGZ x {*r.InMerge(0x0000ffff)})
(RISBGZ (MOVBZreg x) {r}) && r.InMerge(0x000000ff) != nil => (RISBGZ x {*r.InMerge(0x000000ff)})

// Absorb 'rotate then insert selected bits [into zero]' into zero extension.

    }
  }
  return errors::InvalidArgument(
      "Multiple NextIteration inputs to merge node ",
      FormatNodeForError(*merge), ": \n", absl::StrJoin(backedges, "\n"),
      "\nMerge nodes can have at most one incoming NextIteration edge.");
}

Status FindUniqueBackedge(Node* merge, const Edge** result) {
  *result = nullptr;
  CHECK(merge->IsMerge());