Implement ir_if (for if-statments) and conversion from AST

The following tests now pass:

    glslparsertest/shaders/if1.frag
    glslparsertest/shaders/if2.frag

The following tests that used to pass now fail.  It appears that most
of these fail because ast_nequal and ast_equal are not converted to HIR.

    shaders/glsl-unused-varying.frag
    shaders/glsl-fs-sqrt-branch.frag

ir_print_visitor.cpp

@@ -265,3 +265,27 @@ ir_print_visitor::visit(ir_return *ir)
 
    printf(")");
 }
+
+
+void
+ir_print_visitor::visit(ir_if *ir)
+{
+   printf("(if ");
+   ir->condition->accept(this);
+
+   printf("(\n");
+   foreach_iter(exec_list_iterator, iter, ir->then_instructions) {
+      ir_instruction *const inst = (ir_instruction *) iter.get();
+
+      inst->accept(this);
+   }
+   printf(")\n");
+
+   printf("(\n");
+   foreach_iter(exec_list_iterator, iter, ir->else_instructions) {
+      ir_instruction *const inst = (ir_instruction *) iter.get();
+
+      inst->accept(this);
+   }
+   printf("))\n");
+}