[BPF] handle unsigned icmp ops in BPFAdjustOpt pass

When investigating an issue with bcc tool inject.py, I found
a verifier failure with latest clang. The portion of code
can be illustrated as below:
  struct pid_struct {
    u64 curr_call;
    u64 conds_met;
    u64 stack[2];
  };
  struct pid_struct *bpf_map_lookup_elem();
  int foo() {
    struct pid_struct *p = bpf_map_lookup_elem();
    if (!p) return 0;
    p->curr_call--;
    if (p->conds_met < 1 || p->conds_met >= 3)
        return 0;
    if (p->stack[p->conds_met - 1] == p->curr_call)
        p->conds_met--;
    ...
  }

The verifier failure looks like:
  ...
  8: (79) r1 = *(u64 *)(r0 +0)
   R0_w=map_value(id=0,off=0,ks=4,vs=32,imm=0) R10=fp0 fp-8=mmmm????
  9: (07) r1 += -1
  10: (7b) *(u64 *)(r0 +0) = r1
   R0_w=map_value(id=0,off=0,ks=4,vs=32,imm=0) R1_w=inv(id=0) R10=fp0 fp-8=mmmm????
  11: (79) r2 = *(u64 *)(r0 +8)
   R0_w=map_value(id=0,off=0,ks=4,vs=32,imm=0) R1_w=inv(id=0) R10=fp0 fp-8=mmmm????
  12: (bf) r3 = r2
  13: (07) r3 += -3
  14...