selftests/bpf: Tests for is_scalar_branch_taken tnum logic

This patch adds tests for the new jeq and jne logic in
is_scalar_branch_taken. The following shows the first test failing
before the previous patch is applied. Once the previous patch is
applied, the verifier can use the tnum values to deduce that instruction
7 is dead code.

  0: call bpf_get_prandom_u32#7  ; R0_w=scalar()
  1: w0 = w0                     ; R0_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
  2: r0 >>= 30                   ; R0_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=3,var_off=(0x0; 0x3))
  3: r0 <<= 30                   ; R0_w=scalar(smin=0,smax=umax=umax32=0xc0000000,smax32=0x40000000,var_off=(0x0; 0xc0000000))
  4: r1 = r0                     ; R0_w=scalar(id=1,smin=0,smax=umax=umax32=0xc0000000,smax32=0x40000000,var_off=(0x0; 0xc0000000)) R1_w=scalar(id=1,smin=0,smax=umax=umax32=0xc0000000,smax32=0x40000000,var_off=(0x0; 0xc0000000))
  5: r1 += 1024                  ; R1_w=scalar(smin=umin=umin32=1024,smax=umax=umax32=0xc0000400,smin32=0x80000400,smax32=0x40000400,var_off=(0x400; 0xc0000000))
  6: if r1 != r0 goto pc+1       ; R0_w=scalar(id=1,smin=umin=umin32=1024,smax=umax=umax32=0xc0000000,smin32=0x80000400,smax32=0x40000000,var_off=(0x400; 0xc0000000)) R1_w=scalar(smin=umin=umin32=1024,smax=umax=umax32=0xc0000000,smin32=0x80000400,smax32=0x40000400,var_off=(0x400; 0xc0000000))
  7: r10 = 0
  frame pointer is read only

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/550004f935e2553bdb2fb1f09cbde7d0452112d0.1755694148.git.paul.chaignon@gmail.com

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