Put function bodies under function signatures, instead of flat in the parent.

This will let us know the length of function bodies for the purpose of
inlining (among other uses).

ir_constant_folding.cpp

@@ -46,14 +46,14 @@ ir_constant_folding_visitor::visit(ir_variable *ir)
 void
 ir_constant_folding_visitor::visit(ir_label *ir)
 {
-   (void) ir;
+   ir->signature->accept(this);
 }
 
 
 void
 ir_constant_folding_visitor::visit(ir_function_signature *ir)
 {
-   (void) ir;
+   visit_exec_list(&ir->body, this);
 }