Make sure `offset(0)` is handled correctly

[RalfJung]

The offset intrinsic (called inbounds_gep in the codegen_ssa API) is now always defined when the offset is 0, even if the pointer is 0 or dangling.

The GCC backend uses new_array_access to model that; it is important to make sure that if the index is 0, this is never UB.

[antoyo]

Note to self: GIMPLE's TARGET_MEM_REF might be useful for gep implementation.