JIT: generalize ParseArrayAddress a bit more (#112527)

Now that we are stack allocating arrays, the array reference may be either TYP_BYREF
or TYP_I_IMPL, so try and recognize those cases when possible. In particular for OSR
methods the array allocation may "disappear" once we redirect flow to the OSR entry,
so relying on VN is insufficient.

Fixes #112429 (benchmark regression where we're measuring OSR code)

Author: AndyAyersMS

src/coreclr/jit/gentree.cpp

@@ -19887,14 +19887,38 @@ void GenTreeArrAddr::ParseArrayAddress(Compiler* comp, GenTree** pArr, ValueNum*
     ValueNum  vn = comp->GetValueNumStore()->VNLiberalNormalValue(tree->gtVNPair);
     VNFuncApp vnf;
 
+    bool treeIsArrayRef = false;
+
     if (tree->TypeIs(TYP_REF) || comp->GetValueNumStore()->IsVNNewArr(vn, &vnf))
     {
         // This must be the array pointer.
         assert(*pArr == nullptr);
         *pArr = tree;
         assert(inputMul == 1); // Can't multiply the array pointer by anything.
+        treeIsArrayRef = true;
     }
-    else
+    else if (tree->OperIs(GT_LCL_VAR) && tree->TypeIs(TYP_BYREF, TYP_I_IMPL))
+    {
+        // This is sort of like gtGetClassHandle, but that requires TYP_REF
+        //
+        CORINFO_CLASS_HANDLE hnd = comp->lvaGetDesc(tree->AsLclVar())->lvClassHnd;
+
+        if (hnd != NO_CLASS_HANDLE)
+        {
+            DWORD attribs  = comp->info.compCompHnd->getClassAttribs(hnd);
+            treeIsArrayRef = (attribs & CORINFO_FLG_ARRAY) != 0;
+
+            if (treeIsArrayRef)
+            {
+                // This must be the array pointer.
+                assert(*pArr == nullptr);
+                *pArr = tree;
+                assert(inputMul == 1); // Can't multiply the array pointer by anything.
+            }
+        }
+    }
+
+    if (!treeIsArrayRef)
     {
         switch (tree->OperGet())
         {