[rcxdude]

> If the compiler can optimize a piece of code, it can also show the user what it thinks the optimal code would be so that they can rewrite it themselves, if they so choose

This is not straightforward. Apart from the mapping from a several-layers-deep optimization to the source level being very difficult, it may not be even representable in the original language. And even if it is, it may require complicating the code significantly. Part of the point of compiler optimization is so that you can write straightforward code and still have it be fast.

Compilers will often warn on dead code, but only at fairly early stages of translation where it's obvious that something is definitely dead code in all possible contexts and the fix is obvious. These rules are different to what the optimizer actually uses much later on in the pipeline.