By 'those optimisations' I assume you mean the specific rewrites, not superoptimisation? There has been some interesting recent work in rewrite inference, e.g. ruler. (I recall one such project did a survey and found that many—possibly a majority—of the rewrites it had generated automatically were not present in the hand-written corpuses of gcc and clang. I'm not sure if there was subsequent work to supplement gcc and clang; this was a few years ago now; I think they were looking at gcc 7.)