In general ValueHandleBase::ValueIsRAUWd shouldn't be called when not all uses of the value were actually replaced, though, currently formLCSSAForInstructions calls it when it inserts LCSSA-phis.
Calls of ValueHandleBase::ValueIsRAUWd were added to LCSSA specifically to update/invalidate SCEV. In the best case these calls duplicate some of the work already done by SE->forgetValue, though in case when SCEV of the value is SCEVUnknown, SCEV replaces the underlying value of SCEVUnknown with the new value (i.e. acts like LCSSA-phi actually fully replaces the value it is created for), which leads to SCEV being corrupted because LCSSA-phi rarely dominates all uses of its inputs.
This fixes bug https://bugs.llvm.org/show_bug.cgi?id=44058.