[PATCH] Document arithmetic overflow semantics

[Robert Dewar]

> The issue is GCC's internal representations, which shouldn't suffer
> from the same issue of poorly defined languages.  Hence, if CSE sees
> "x = i++; i = x" it had better understand exactly what "i = i++" means
> at the RTL-level.  One might hope that the interpretation of RTL and
> TREEs are independent of the source language and target architecture.

Most certainly the meaning should be specified and exact, but also it can
most certainly be non-deterministic with several possbile outcomes.