[NewGVN] Unreachable if block in performSymbolicLoadEvaluation (NFC)

[ManuelJBrito]

Summary

When performing symbolic load evaluation we currently check if the defining instruction is unreachable. AFAICT this is not possible because the defining access is a clobber of the load, therefore the defining instruction dominates the load. In NewGVN if we are processing an instruction its because we have deemed it reachable so it's not possible to have an unreachable dominator.

[asbirlea]

Can you replace this with an assert instead?

[ManuelJBrito]

Add assert, as per Alina's request.

[kmitropoulou]

LGTM!

llvm/lib/Transforms/Scalar/NewGVN.cpp

if (isa<UndefValue>(LoadAddressLeader))
     return createConstantExpression(PoisonValue::get(LI->getType()));
   MemoryAccess *OriginalAccess = getMemoryAccess(I);
   MemoryAccess *DefiningAccess =
       MSSAWalker->getClobberingMemoryAccess(OriginalAccess);
 
   if (!MSSA->isLiveOnEntryDef(DefiningAccess)) {
     if (auto *MD = dyn_cast<MemoryDef>(DefiningAccess)) {
       Instruction *DefiningInst = MD->getMemoryInst();
-      // If the defining instruction is not reachable, replace with poison.
-      if (!ReachableBlocks.count(DefiningInst->getParent()))
-        return createConstantExpression(PoisonValue::get(LI->getType()));
+      
       // This will handle stores and memory insts.  We only do if it the
       // defining access has a different type, or it is a pointer produced by
       // certain memory operations that cause the memory to have a fixed value
       // (IE things like calloc).
       if (const auto *CoercionResult =
               performSymbolicLoadCoercion(LI->getType(), LoadAddressLeader, LI,
                                           DefiningInst, DefiningAccess))
         return CoercionResult;