[Mem2Reg] !nonnull and !noundef are enough for creating assume

[StephenFan]

Summary

The behavior of violating !nonnull and !noundef is immediate undefined
behavior, that's enough to create a non-null assumption.

[nikic]

The isKnownNonZero() check ensures that no assumptions are generated for facts that are already known through other means anyway. This will result in the generation of many unnecessary assumes.

llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp

 static void convertMetadataToAssumes(LoadInst *LI, Value *Val,
                                      const DataLayout &DL, AssumptionCache *AC,
                                      const DominatorTree *DT) {
   // If the load was marked as nonnull we don't want to lose that information
   // when we erase this Load. So we preserve it with an assume. As !nonnull
   // returns poison while assume violations are immediate undefined behavior,
   // we can only do this if the value is known non-poison.
   if (AC && LI->getMetadata(LLVMContext::MD_nonnull) &&
-      LI->getMetadata(LLVMContext::MD_noundef) &&
-      !isKnownNonZero(Val, DL, 0, AC, LI, DT))
+      LI->getMetadata(LLVMContext::MD_noundef))
     addAssumeNonNull(AC, LI);
 }
 
 static void removeIntrinsicUsers(AllocaInst *AI) {
   // Knowing that this alloca is promotable, we know that it's safe to kill all
   // instructions except for load and store.
 
   for (Use &U : llvm::make_early_inc_range(AI->uses())) {