selftests/bpf: Add a verifier scale test with unknown bounded loop

The original bcc pull request https://github.com/iovisor/bcc/pull/3270 exposed
a verifier failure with Clang 12/13 while Clang 4 works fine.

Further investigation exposed two issues:

  Issue 1: LLVM may generate code which uses less refined value. The issue is
           fixed in LLVM patch: https://reviews.llvm.org/D97479



  Issue 2: Spills with initial value 0 are marked as precise which makes later
           state pruning less effective. This is my rough initial analysis and
           further investigation is needed to find how to improve verifier
           pruning in such cases.

With the above LLVM patch, for the new loop6.c test, which has smaller loop
bound compared to original test, I got:

  $ test_progs -s -n 10/16
  ...
  stack depth 64
  processed 390735 insns (limit 1000000) max_states_per_insn 87
      total_states 8658 peak_states 964 mark_read 6
  #10/16 loop6.o:OK

Use the original loop bound, i.e., commenting out "#define WORKAROUND", I got:

  $ test_progs -s -n 10/16
  ...
  BPF program is too large. Processed 1000001 insn
  stack depth 64
  processed 1000001 insns (limit 1000000) max_states_per_insn 91
      total_states 23176 peak_states 5069 mark_read 6
  ...
  #10/16 loop6.o:FAIL

The purpose of this patch is to provide a regression test for the above LLVM fix
and also provide a test case for further analyzing the verifier pruning issue.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Zhenwei Pi <pizhenwei@bytedance.com>
Link: https://lore.kernel.org/bpf/20210226223810.236472-1-yhs@fb.com