JIT: stop tracking TYP_I_IMPL locals in escape analysis (#114130)

Abstractly a non-GC local can't cause escape. On 32 bit platforms tracking connections
to `TYP_I_IMPL` leads to confusion once we start struct field tracking, as we logically
"overlay" GC and non-GC fields.

Contributes to #104936

Author: AndyAyersMS

src/coreclr/jit/objectalloc.cpp

@@ -71,7 +71,7 @@ ObjectAllocator::ObjectAllocator(Compiler* comp)
 //
 bool ObjectAllocator::IsTrackedType(var_types type)
 {
-    const bool isTrackableScalar = (type == TYP_REF) || (genActualType(type) == TYP_I_IMPL) || (type == TYP_BYREF);
+    const bool isTrackableScalar = (type == TYP_REF) || (type == TYP_BYREF);
     return isTrackableScalar;
 }
 
@@ -1579,12 +1579,19 @@ bool ObjectAllocator::CanLclVarEscapeViaParentStack(ArrayStack<GenTree*>* parent
 
         switch (parent->OperGet())
         {
-            // Update the connection graph if we are storing to a local.
-            // For all other stores we mark the local as escaping.
             case GT_STORE_LCL_VAR:
             {
-                // Add an edge to the connection graph.
                 const unsigned int dstLclNum = parent->AsLclVar()->GetLclNum();
+
+                // If we're not tracking stores to this local, the value
+                // does not escape.
+                if (!IsTrackedLocal(dstLclNum))
+                {
+                    canLclVarEscapeViaParentStack = false;
+                    break;
+                }
+
+                // Add an edge to the connection graph.
                 const unsigned int srcLclNum = lclNum;
 
                 AddConnGraphEdge(dstLclNum, srcLclNum);
@@ -1626,13 +1633,25 @@ bool ObjectAllocator::CanLclVarEscapeViaParentStack(ArrayStack<GenTree*>* parent
             case GT_COLON:
             case GT_QMARK:
             case GT_ADD:
-            case GT_SUB:
             case GT_FIELD_ADDR:
                 // Check whether the local escapes via its grandparent.
                 ++parentIndex;
                 keepChecking = true;
                 break;
 
+            case GT_SUB:
+                // Sub of two GC refs is no longer a GC ref.
+                if (!parent->TypeIs(TYP_BYREF, TYP_REF))
+                {
+                    canLclVarEscapeViaParentStack = false;
+                    break;
+                }
+
+                // Check whether the local escapes higher up
+                ++parentIndex;
+                keepChecking = true;
+                break;
+
             case GT_BOX:
                 isCopy = wasCopy;
                 ++parentIndex;
@@ -1787,7 +1806,6 @@ void ObjectAllocator::UpdateAncestorTypes(GenTree* tree, ArrayStack<GenTree*>* p
                 FALLTHROUGH;
             case GT_QMARK:
             case GT_ADD:
-            case GT_SUB:
             case GT_FIELD_ADDR:
             case GT_INDEX_ADDR:
             case GT_BOX:
@@ -1799,6 +1817,33 @@ void ObjectAllocator::UpdateAncestorTypes(GenTree* tree, ArrayStack<GenTree*>* p
                 keepChecking = true;
                 break;
 
+            case GT_SUB:
+            {
+                // Parent type can be TYP_I_IMPL, TYP_BYREF.
+                // But not TYP_REF.
+                //
+                var_types parentType = parent->TypeGet();
+                assert(parentType != TYP_REF);
+
+                // New type can be TYP_I_IMPL, TYP_BYREF.
+                // But TYP_BYREF only if parent is also
+                //
+                if (parentType != newType)
+                {
+                    // We must be retyping TYP_BYREF to TYP_I_IMPL.
+                    //
+                    assert(newType == TYP_I_IMPL);
+                    assert(parentType == TYP_BYREF);
+                    parent->ChangeType(newType);
+
+                    // Propgate that upwards.
+                    //
+                    ++parentIndex;
+                    keepChecking = true;
+                }
+                break;
+            }
+
             case GT_COLON:
             {
                 GenTree* const lhs = parent->AsOp()->gtGetOp1();