perf report: Allow hierarchy mode for --children

perf report: Allow hierarchy mode for --children

It was prohibited because the output fields in the children mode were
not handled properly with hierarchy.  But we can have the output fields
in the same level, it can allow them together.

For example, latency mode adds more output fields by default and now
they are displayed properly.

  $ perf record --latency -g -- perf test -w thloop

  $ perf report -H --stdio
  # To display the perf.data header info, please use --header/--header-only options.
  #
  #
  # Total Lost Samples: 0
  #
  # Samples: 2K of event 'cycles:Pu'
  # Event count (approx.): 8266456478
  #
  #       Children  Latency  Overhead   Latency  Command / Shared Object / Symbol
  # ...........................................  ........................................................
  #
       0.08%    0.16%   100.00%   100.00%        perf
          0.08%    0.16%     0.24%     0.47%        ld-linux-x86-64.so.2
             0.12%    0.24%     0.12%     0.24%        [.] _dl_relocate_object
             0.08%    0.16%     0.08%     0.16%        [.] _dl_lookup_symbol_x
             0.03%    0.06%     0.03%     0.06%        [.] strcmp
             0.00%    0.01%     0.00%     0.01%        [.] _dl_start
             0.00%    0.00%     0.00%     0.00%        [.] _dl_start_user
             0.00%    0.00%     0.00%     0.00%        [.] _dl_sysdep_start
             0.00%    0.00%     0.00%     0.00%        [.] _start
             0.00%    0.00%     0.00%     0.00%        [.] dl_main
          0.03%    0.06%     0.03%     0.06%        libLLVM-16.so.1
             0.03%    0.06%     0.03%     0.06%        [.] llvm::StringMapImpl::RehashTable(unsigned int)
             0.00%    0.00%     0.00%     0.00%        [.] 0x00007f137ccd18e8
          0.00%    0.00%    99.66%    99.31%        perf
            99.66%   99.31%    99.66%    99.31%        [.] test_loop
              |
              |--49.86%--0x7f137b633d68
              |          0x55dbdbbb7d2c
              ...

Link: https://lore.kernel.org/r/20250307080829.354947-2-namhyung@kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>