WIP: replace loop controls with a bound type and max_iterations count.
glsl: Remove unused fields iv_scale and biv from loop_variable class.
glsl: Remove unnecessary list walk from loop_control_visitor.

When loop_control_visitor::visit_leave(ir_loop *) is analyzing a loop
terminator that acts on a certain ir_variable, it doesn't need to walk
the list of induction variables to find the loop_variable entry
corresponding to the variable.  It can just look it up in the
loop_variable_state hashtable and verify that the loop_variable entry
represents an induction variable.
WIP
glsl/loops: Allocate loop_terminator using new(mem_ctx) syntax.

Patches to follow will introduce code into the loop_terminator
constructor.  Allocating loop_terminator using new(mem_ctx) syntax
will ensure that the constructor runs.
WIP
WIP