glsl: Remove ir_call::get_callee() and set_callee().

Previously, set_callee() performed some assertions about the type of the
ir_call; protecting the bare pointer ensured these checks would be run.

However, ir_call no longer has a type, so the getter and setter methods
don't actually do anything useful.  Remove them in favor of accessing
callee directly, as is done with most other fields in our IR.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/opt_constant_folding.cpp

@@ -118,7 +118,7 @@ ir_visitor_status
 ir_constant_folding_visitor::visit_enter(ir_call *ir)
 {
    /* Attempt to constant fold parameters */
-   exec_list_iterator sig_iter = ir->get_callee()->parameters.iterator();
+   exec_list_iterator sig_iter = ir->callee->parameters.iterator();
    foreach_iter(exec_list_iterator, iter, *ir) {
       ir_rvalue *param_rval = (ir_rvalue *)iter.get();
       ir_variable *sig_param = (ir_variable *)sig_iter.get();