Add mem unexpected behaviour

[NeuralCoder3]

The add_mem phase behaves differently, depending on surrounding externals.
Even if these externals are never called and to not interfere with the functions.
Example: https://github.com/NeuralCoder3/thorin2/blob/ho_codegen/lit/mem/add_top_mem.thorin
Example2: https://github.com/NeuralCoder3/thorin2/blob/ho_codegen/lit/mem/add_top_mem2.thorin

In add_top_mem, the call remains f (⊤:%mem.M, argc_1111242, return_1111250).
In add_top_mem, the call is changed to f (_1113823, argc_1113827, return_1113835).

The only difference between the two examples are the externals g and h (h seems to be the important one).

It seems like mems in application argument position are sometimes taken in consideration as current memory.
However, we should only consider vars and application results.

[NeuralCoder3]

One problem was that the else case where mem as result from any operation is registered in val2mem.
I think we do not need this case.

But this does not explain why other externals influence the result.

[NeuralCoder3]

The behavior was probably caused due to the sharing of the two ⊤:%mem.M instances.
The fix is in the ho_codegen branch (the only place where it is currently needed) and will be merged with it.