NotVar applied recursively also for SYSTEM.VAL. Refs: #165.
The meaning is: if SYSTEM.VAL is applied to something that is not a variable, then
the result is also not a variable.
Such a case is hard to produce because the compiler normally does not generate a SYSTEM.VAL node
when converting a pointer. However, there are (rarely used) options that turn on the generation
of SYSTEM.VAL nodes in such cases.
Using NotVar recursively makes the code easier to read and also covers such exotic cases.

Signed-off-by: Josef Templ <josef.templ@gmail.com>