bpf: compute SCCs in program control flow graph

Compute strongly connected components in the program CFG.
Assign an SCC number to each instruction, recorded in
env->insn_aux[*].scc. Use Tarjan's algorithm for SCC computation
adapted to run non-recursively.

For debug purposes print out computed SCCs as a part of full program
dump in compute_live_registers() at log level 2, e.g.:

  func#0 @0
  Live regs before insn:
        0: .......... (b4) w6 = 10
    2   1: ......6... (18) r1 = 0xffff88810bbb5565
    2   3: .1....6... (b4) w2 = 2
    2   4: .12...6... (85) call bpf_trace_printk#6
    2   5: ......6... (04) w6 += -1
    2   6: ......6... (56) if w6 != 0x0 goto pc-6
        7: .......... (b4) w6 = 5
    1   8: ......6... (18) r1 = 0xffff88810bbb5567
    1  10: .1....6... (b4) w2 = 2
    1  11: .12...6... (85) call bpf_trace_printk#6
    1  12: ......6... (04) w6 += -1
    1  13: ......6... (56) if w6 != 0x0 goto pc-6
       14: .......... (b4) w0 = 0
       15: 0......... (95) exit
   ^^^
  SCC number for the instruction

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250611200836.4135542-2-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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