[ValueTracking] isGuaranteedNotToBePoison should return true on undef

This is a one-line fix to isGuaranteedNotToBePoison to return true if
undef is given.

Author: aqjune

llvm/lib/Analysis/ValueTracking.cpp

@@ -4888,7 +4888,7 @@ static bool isGuaranteedNotToBeUndefOrPoison(const Value *V,
 
   if (auto *C = dyn_cast<Constant>(V)) {
     if (isa<UndefValue>(C))
-      return PoisonOnly;
+      return PoisonOnly && !isa<PoisonValue>(C);
 
     if (isa<ConstantInt>(C) || isa<GlobalVariable>(C) || isa<ConstantFP>(V) ||
         isa<ConstantPointerNull>(C) || isa<Function>(C))

llvm/unittests/Analysis/ValueTrackingTest.cpp

@@ -884,6 +884,10 @@ TEST_F(ValueTrackingTest, isGuaranteedNotToBeUndefOrPoison) {
                 "  ret void\n"
                 "}\n");
   EXPECT_EQ(isGuaranteedNotToBeUndefOrPoison(A), true);
+  EXPECT_EQ(isGuaranteedNotToBeUndefOrPoison(UndefValue::get(IntegerType::get(Context, 8))), false);
+  EXPECT_EQ(isGuaranteedNotToBeUndefOrPoison(PoisonValue::get(IntegerType::get(Context, 8))), false);
+  EXPECT_EQ(isGuaranteedNotToBePoison(UndefValue::get(IntegerType::get(Context, 8))), true);
+  EXPECT_EQ(isGuaranteedNotToBePoison(PoisonValue::get(IntegerType::get(Context, 8))), false);
 }
 
 TEST_F(ValueTrackingTest, isGuaranteedNotToBeUndefOrPoison_assume) {