PTA FI: identify GEP instructions and preprocess UNKNOWN_OFFSETS

[mchalupa]

if we have code like this:

int *p = array[0];
p++;

Then in flow-insensitive analysis we end up with UNKNOWN_OFFSET even though there's no loop. Would it be possible to identify those parts and set the UNKNOWN_OFFSET immediately?

[mchalupa]

Done for FI, but what about for FS?