fix Issue 10526 - opDispatch with IFTI should not disable UFCS

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.
Merge pull request #2292 from 9rnsr/fix_ufcs

Issue 10526 - opDispatch with IFTI should not disable UFCS