[Bug middle-end/85598] [7/8/9 Regression] Incorrect warning only at -O2 and -O3

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85598

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
The problem is that the warning pass sees x as having the range of [0, 256]
rather than [0, 255].  The incorrect range can also be seen in EVRP.  There is
no warning at -O1 because the range determined at that level is that of
unsigned int which is considered to be the same as unknown.  The warning does
not trigger for such ranges.