[Bug c/91373] gcc6.2.0: ((U32)((U16 * U16)) >> 31) cannot always get correct result with gcc -O2

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91373

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And Bugzilla asks you to read https://gcc.gnu.org/bugs before creating a new
bug, and that page asks you to try -fsanitize=undefined to see if your code is
undefined. If you'd done that you'd have been told your code is undefined.