This should be mostly a noop, except that a plain dereference of a
variable that is not part of a constant expression could now get
"constant folded". I expect that for all current backends this will
be either a noop, or possibly a win when it provokes more
ir_algebraic. It'll also ensure that when new features are added,
tree walking will work normally. Before this, constants weren't
getting folded inside of loops.
