[Bug middle-end/56917] -ftrapv detects a overflow wrongly.

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56917

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
fold looks too much through sign-changing NOP conversions, this makes it
tricky to spot errors in patterns.