From what I can glean, it'll and I've exist as standard contractions, but I am unsure of whether it'll've either exists or is acceptable. "It will have" should be able to be reduced to "it'll've", ...
Is the contraction from that will to that’ll an actual word or not?
Are poetic contractions, such as "e'er", "o'er" and "ne'er" (and other less common ones), English? As in officially recognized?
Google finds 52,000,000 matches for ain't but non-natives simply can't look up this word. Wiktionary isn't helpful. Is it some kind of 'wildcard' for "am/is/are not"?