glsl: Convert constant folding to the rvalue visitor.

glsl: Convert constant folding to the rvalue visitor.

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.