glsl/loops: Get rid of lower_bounded_loops and ir_loop::normative_bound.

Now that loop_controls no longer creates normatively bound loops,
there is no need for ir_loop::normative_bound or the
lower_bounded_loops pass.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/ir.cpp

@@ -1277,7 +1277,6 @@ ir_constant::is_basis() const
 ir_loop::ir_loop()
 {
    this->ir_type = ir_type_loop;
-   this->normative_bound = -1;
 }