[ValueTracking] A value is never undef or poison if it must raise UB

[aqjune]

Summary

This patch helps isGuaranteedNotToBeUndefOrPoison return true if the value
makes the program always undefined.

According to value tracking functions' comments, it is not still in consensus
whether a poison value can be bitwise or not, so conservatively only the case with
i1 is considered.

[aqjune]

Rebase

[nlopes]

LGTM

llvm/lib/Analysis/ValueTracking.cpp

bool llvm::isGuaranteedNotToBeUndefOrPoison(const Value *V) {
 
   if (auto II = dyn_cast<ICmpInst>(V)) {
     if (llvm::all_of(II->operands(), [](const Value *V) {
           return isGuaranteedNotToBeUndefOrPoison(V);
         }))
       return true;
   }
 
+  if (auto I = dyn_cast<Instruction>(V)) {
+    if (programUndefinedIfFullPoison(I) && I->getType()->isIntegerTy(1))
+      // Note: once we have an agreement that poison is a value-wise concept,
+      // we can remove the isIntegerTy(1) constraint.
+      return true;
+  }
+
   return false;
 }
 
 OverflowResult llvm::computeOverflowForSignedAdd(const AddOperator *Add,
                                                  const DataLayout &DL,
                                                  AssumptionCache *AC,
                                                  const Instruction *CxtI,
                                                  const DominatorTree *DT) {