Sign extension generates BOT

[elazarg]

This seems to happen on numbers that are close to the bound of the extension.

The test ebpf-samples/cilium/bpf_netdev.o from-netdev should be marked as failing:

$ ./check --no-simplify ebpf-samples/cilium/bpf_netdev.o from-netdev -v 
...
Pre-invariant : [...
     r1.type=number, r1.svalue=[0, 4294967161], r1.svalue=r1.uvalue, ..., r1.uvalue=[0, 4294967161], ...]
499:
  assert r1.type == number;
  r1 s32= r1;
  goto 501;

Post-invariant: _|_
...

This blocks #678.