Add optimizer pass to avoid copies of variables no longer used

[bbannier]

Most of them time when generating code we do not know whether a HILTI value will be used later and consequentially do not use std::move when emitting C++ code. This means that we might that we could e.g., needlessly create copies when calling functions which parameters pass by copy, or create copies when storing results in another variable.

The work in #1690 should gives us control flow information to decide whether a reference to a variable (a use of a hilti::ID referring to a variable) is the last use. If we persisted that information with e.g., the expression we could make use of it during C++ codegen.