[Bug middle-end/78115] Missed optimization for "int modulo 2^31"

["glisse at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78115

--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> ---
For the first part, when we transform (X+C1)+C2 to X+(C1+C2), we check that
C1+C2 doesn't overflow. But if C1+C2 would give INT_MIN, we still have the
possibility to generate X-INT_MIN without going to an unsigned type.

For the second part, I am not sure either. Some guesses:
X - INT_MIN becomes X ^ INT_MIN. Since we know that all the bits set in INT_MIN
(the top one) are also set in X (< 0), this becomes X & INT_MAX. In the branch
X >= 0, X & INT_MAX would fold to X (kind of like a neutral element) so we can
do the operation unconditionally.
A bit far-fetched maybe...