It's undefined behavior, so the compiler is free to do pretty much anything it wants. It can always assume it's true; it can always assume it's false; it can omit code to return true 50% of the time.
In theory it can do whatever it wants. In practice, it generally just assumes undefined behavior won't happen, and will therefore assume that any conditions that could lead to undefined behavior are false (and prune any dead code that results from those assumptions).
The other thing that can happen is LLVM has the concept of a undefined value, which is distinct from undefined behavior. Undefined values may be unknown, but the compiler can assume that any possible value still results in defined behavior, and optimize accordingly. As an example, an un-initialized stack variable has an undefined value, but various operations on it may still result in defined behavior regardless of the value.
