Fix VNZeroForType's handling of SIMD types

Previously this codepath was reachable, but did not matter as
the returned values were immediately discarded because of the
conservative logic in VNApplySelectorsTypeCheck. This is still
more or less the case, but let's just fix it properly.

Author: SingleAccretion

src/coreclr/jit/valuenum.cpp

@@ -1814,7 +1814,12 @@ ValueNum ValueNumStore::VNZeroForType(var_types typ)
         case TYP_SIMD12:
         case TYP_SIMD16:
         case TYP_SIMD32:
-            return VNForLongCon(0);
+            // We do not have the base type - a "fake" one will have to do. Note that we cannot
+            // use the HWIntrinsic "get_Zero" VNFunc here. This is because they only represent
+            // "fully zeroed" vectors, and here we may be loading one from memory, leaving upper
+            // bits undefined. So using "SIMD_Init" is "the next best thing", so to speak, and
+            // TYP_FLOAT is one of the more popular base types, so that's why we use it here.
+            return VNForFunc(typ, VNF_SIMD_Init, VNForFloatCon(0), VNForSimdType(genTypeSize(typ), TYP_FLOAT));
 #endif // FEATURE_SIMD
 
         // These should be unreached.
@@ -1860,6 +1865,17 @@ ValueNum ValueNumStore::VNOneForType(var_types typ)
     }
 }
 
+#ifdef FEATURE_SIMD
+ValueNum ValueNumStore::VNForSimdType(unsigned simdSize, var_types simdBaseType)
+{
+    ValueNum baseTypeVN = VNForIntCon(INT32(simdBaseType));
+    ValueNum sizeVN     = VNForIntCon(simdSize);
+    ValueNum simdTypeVN = VNForFunc(TYP_REF, VNF_SimdType, sizeVN, baseTypeVN);
+
+    return simdTypeVN;
+}
+#endif // FEATURE_SIMD
+
 class Object* ValueNumStore::s_specialRefConsts[] = {nullptr, nullptr, nullptr};
 
 //----------------------------------------------------------------------------------------
@@ -9510,9 +9526,7 @@ void Compiler::fgValueNumberSimd(GenTree* tree)
 
         if (encodeResultType)
         {
-            ValueNum vnSize     = vnStore->VNForIntCon(simdNode->GetSimdSize());
-            ValueNum vnBaseType = vnStore->VNForIntCon(INT32(simdNode->GetSimdBaseType()));
-            ValueNum simdTypeVN = vnStore->VNForFunc(TYP_REF, VNF_SimdType, vnSize, vnBaseType);
+            ValueNum simdTypeVN = vnStore->VNForSimdType(simdNode->GetSimdSize(), simdNode->GetSimdBaseType());
             resvnp.SetBoth(simdTypeVN);
 
 #ifdef DEBUG
@@ -9627,9 +9641,8 @@ void Compiler::fgValueNumberHWIntrinsic(GenTree* tree)
 
     if (encodeResultType)
     {
-        ValueNum vnSize     = vnStore->VNForIntCon(hwIntrinsicNode->GetSimdSize());
-        ValueNum vnBaseType = vnStore->VNForIntCon(INT32(hwIntrinsicNode->GetSimdBaseType()));
-        ValueNum simdTypeVN = vnStore->VNForFunc(TYP_REF, VNF_SimdType, vnSize, vnBaseType);
+        ValueNum simdTypeVN =
+            vnStore->VNForSimdType(hwIntrinsicNode->GetSimdSize(), hwIntrinsicNode->GetSimdBaseType());
         resvnp.SetBoth(simdTypeVN);
 
 #ifdef DEBUG

src/coreclr/jit/valuenum.h

@@ -463,6 +463,11 @@ class ValueNumStore
     // It returns NoVN for a "typ" that has no one value, such as TYP_REF.
     ValueNum VNOneForType(var_types typ);
 
+#ifdef FEATURE_SIMD
+    // A helper function for constructing VNF_SimdType VNs.
+    ValueNum VNForSimdType(unsigned simdSize, var_types simdBaseType);
+#endif // FEATURE_SIMD
+
     // Create or return the existimg value number representing a singleton exception set
     // for the the exception value "x".
     ValueNum VNExcSetSingleton(ValueNum x);