Add support for hoisting allocate statements

[arporter]

Allocates within compute code are OK on the CPU but when adding directives to accelerate the code on a GPU, these allocates are left on the CPU and then (with managed memory) copied over to the GPU leading to a lot of unnecessary data traffic. Ideally we should be able to transform these allocates in a similar manner to what we do for automatic arrays. (i.e. make them module scope if they aren't already and then only reallocate them if their size changes).

[sergisiso]

This also happens in at least 1 place of the NEMO 4.0 metoffice configuration and the profiler shows lots of data traffic there. So improving this will also benefit our NEMO 4.0 gpu performance.