[ValueTracking] Return poison for zero-sized types (#122647)

Return `poison` for zero-sized types in `isBitwiseValue`.

Author: pedroclobo

llvm/lib/Analysis/ValueTracking.cpp

@@ -6192,9 +6192,9 @@ Value *llvm::isBytewiseValue(Value *V, const DataLayout &DL) {
   if (isa<UndefValue>(V))
     return UndefInt8;
 
-  // Return Undef for zero-sized type.
+  // Return poison for zero-sized type.
   if (DL.getTypeStoreSize(V->getType()).isZero())
-    return UndefInt8;
+    return PoisonValue::get(Type::getInt8Ty(Ctx));
 
   Constant *C = dyn_cast<Constant>(V);
   if (!C) {

llvm/test/Transforms/MemCpyOpt/store-to-memset-is-nonzero-type.ll

@@ -5,7 +5,7 @@
 
 define void @array_zero(ptr %p) {
 ; CHECK-LABEL: @array_zero(
-; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 1 [[P:%.*]], i8 undef, i64 0, i1 false)
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 1 [[P:%.*]], i8 poison, i64 0, i1 false)
 ; CHECK-NEXT:    ret void
 ;
   store [0 x i8] zeroinitializer, ptr %p
@@ -25,7 +25,7 @@ define void @array_nonzero(ptr %p) {
 
 define void @struct_zero(ptr %p) {
 ; CHECK-LABEL: @struct_zero(
-; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 1 [[P:%.*]], i8 undef, i64 0, i1 false)
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 1 [[P:%.*]], i8 poison, i64 0, i1 false)
 ; CHECK-NEXT:    ret void
 ;
   store { } zeroinitializer, ptr %p

llvm/unittests/Analysis/ValueTrackingTest.cpp

@@ -2869,15 +2869,15 @@ const std::pair<const char *, const char *> IsBytewiseValueTests[] = {
         "ptr inttoptr (i96 -1 to ptr)",
     },
     {
-        "i8 undef",
+        "i8 poison",
         "[0 x i8] zeroinitializer",
     },
     {
         "i8 undef",
         "[0 x i8] undef",
     },
     {
-        "i8 undef",
+        "i8 poison",
         "[5 x [0 x i8]] zeroinitializer",
     },
     {
@@ -2959,15 +2959,15 @@ const std::pair<const char *, const char *> IsBytewiseValueTests[] = {
         "[2 x i16] [i16 -21836, i16 -21846]]",
     },
     {
-        "i8 undef",
+        "i8 poison",
         "{ } zeroinitializer",
     },
     {
         "i8 undef",
         "{ } undef",
     },
     {
-        "i8 undef",
+        "i8 poison",
         "{ {}, {} } zeroinitializer",
     },
     {