Optimise treatment of local variables whose address has not been taken

[cp526]

As suggested by Peter.

Cerberus and CN treat C's local mutable variables the same as global heap allocations, leading to a nice uniform treatment. However, for local variables whose address is not taken there is room for optimisation, as this treatment incurs extra performance cost: CN has to track alignment constraints for pointers, check accesses are aligned, track the disjointness of their resources from other resources, etc.

We should optimise the handling of these variables:

• propagate from Cabs/Ail the information about whether the address of a local variable is ever taken down to Core and CN
• skip unnecessary checks on those variables
• skip unnecessary recording of constraints for these variables.