C++ API for collecting expression indices of variable uses in MLIL

[mostobriv]

What is the feature you'd like to have?
Not to replace existing one, but to add an additional api so we can get the instruction or expression indices, depending on the case.

Additional Information:
I guess title speaks for itself, sometimes it's helpful due to various circumstances in which mlil screwing up the optimization part and splitting expressions.

This request is kinda related to #2739, but instead of unification of var_uses logic, i think the better is to have both APIs