...with one option for undefined behaviour being for it to behave "in some documented manner characteristic of the platform" (C11 para 3.4.3.2). Which is not the same thing as being implementation defined, of course, but is at least a step above simply deciding your program is invalid and generating bogus code because of it.

So compilers could do this stuff already. I have no idea why they don't. C isn't really the language for people who want platform independence.

Well sure. They don't because you can get better performance if you are able to eliminate branches that you can prove aren't taken.