Issue 10526 - opDispatch with IFTI should not disable UFCS

http://d.puremagic.com/issues/show_bug.cgi?id=10526

When opDispatch result is a function template that requires IFTI, it would be kept on `DotTemplateInstanceExp` so `ti->needsTypeInference(sc)` returns TRUE in its semantic().

UFCS dispatch should check that the opDispatch result is really valid or not, by instantiating the function template.