This adjusts mayHaveSideEffect() to return true for !willReturn() instructions. Just like other side-effects, non-willreturn calls (aka "divergence") cannot be removed and cannot be reordered relative to other side effects. This fixes a number of bugs where non-willreturn calls are either incorrectly dropped or moved. In particular, it also fixes the last open problem in https://bugs.llvm.org/show_bug.cgi?id=50511.
I performed a cursory review of all current mayHaveSideEffect() uses, which convinced me that these are indeed the desired semantics. Places that do not want to consider non-willreturn as a sideeffect generally do not want mayHaveSideEffect() semantics at all. I identified two such cases, which are addressed by D106591 and D106742. Finally, there is a use in SCEV for which we don't really have an appropriate API right now -- what it wants is basically "would this be considered forward progress". I've just spelled out the previous semantics there.