glsl/loops: Remove unnecessary list walk from loop_control_visitor.

glsl/loops: 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.

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