[Bug tree-optimization/81388] Incorrect code generation with -O1 -fno-strict-overflow

[terrelln at fb dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81388

--- Comment #4 from Nick Terrell <terrelln at fb dot com> ---
Good point Marc.

The generated assembly is wrong because the loop should be run once, but it is
run 2^63  times. Changing the amount dst is incremented by to 4 makes it run
2^62 times, and changing it to 8 makes it run 2^61 times.