selftests/bpf: Test widen_imprecise_scalars() with different stack depth

A test case for a situation when widen_imprecise_scalars() is called
with old->allocated_stack > cur->allocated_stack. Test structure:

    def widening_stack_size_bug():
      r1 = 0
      for r6 in 0..1:
        iterator_with_diff_stack_depth(r1)
        r1 = 42

    def iterator_with_diff_stack_depth(r1):
      if r1 != 42:
        use 128 bytes of stack
      iterator based loop

iterator_with_diff_stack_depth() is verified with r1 == 0 first and
r1 == 42 next. Causing stack usage of 128 bytes on a first visit and 8
bytes on a second. Such arrangement triggered a KASAN error in
widen_imprecise_scalars().

Signed-off-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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