Similar to other situations where the visitor pattern doesn't fit, in
this case we need the pointer to the base instruction in the
instruction stream for where to insert any new instructions we
generate (not the instruction in the tree we're looking at). By
removing the code for setting the base_ir, flattened expressions would
end up, for example, before the function definition where they had appeared.
Create separate subclasses of ir_dereference for variable, array, and
record dereferences. As a side effect, array and record dereferences
no longer point to ir_variable objects directly. Instead they each
point to an ir_dereference_variable object.
This is the first of several steps in the refactoring process. The
intention is that ir_dereference will eventually become an abstract
base class.
