ISO C99 actually defines multiple types of deviating behaviour. What you're describing is closer to implementation-defined behaviour than anything else.
The three behaviours relevant in this discussion, from section 3.4:
3.4.1 implementation-defined behavior
unspecified behavior where each implementation documents how the choice is made
EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right.
3.4.3 undefined behavior
behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements
Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).
An example of undefined behavior is the behavior on integer overflow.
3.4.4 unspecified behavior
behavior where this International Standard provides two or more possibilities and imposes no further requirements on which is chosen in any instance
An example of unspecified behavior is the order in which the arguments to a function are evaluated.
Using integer overflow, as in your example, for optimization has been shown to be beneficial by Charles Carruth in a talk he did at CppCon in 2016.[1] I think it would be best to have something similar to Zig's wrapping and saturating addition operators instead, but for that I think it is better to just use Zig (which I personally am very willing to do once they reach 1.0 and other compiler implementations are available).[2]
[1] https://youtu.be/yG1OZ69H_-o?si=x-9ALB8JGn5Qdjx_&t=2357 [2] https://ziglang.org/documentation/0.15.2/#Operators