Complete speculation, but it could be simply that clang only considers applying the branchless optimisation when the code inside the "if" block "looks like" a single instruction/statement -- and this "looks like" check might be done quite early, before "*x++ = y;" and "*x = y; x++;" are converted to the same thing.

Why restrict the optimisation unnecessarily like this? It's never the intention to artificially restrict optimisations, but they are difficult to test and debug as they can interact with other optimisations, and applying an optimisation too broadly leads to horrible correctness bugs. So if you cannot be certain that you understand all possible interactions now and in the future, it makes some sense to be conservative in applying them.