reflect: track flags when size changes from fits-in-pointer to not

Fixes #3328

testdata/reflect.go

@@ -129,6 +129,7 @@ func main() {
 		&linkedList{
 			foo: 42,
 		},
+		struct{ A, B uintptr }{2, 3},
 		// interfaces
 		[]interface{}{3, "str", -4 + 2.5i},
 	} {

testdata/reflect.txt

@@ -334,6 +334,20 @@ reflect type: ptr
     exported: false
     reflect type: int addrable=true caninterface=false
       int: 42
+reflect type: struct
+  struct: 2
+  field: 0 A
+  tag: 
+  embedded: false
+  exported: true
+  reflect type: uintptr
+    uint: 2
+  field: 1 B
+  tag: 
+  embedded: false
+  exported: true
+  reflect type: uintptr
+    uint: 3
 reflect type: slice comparable=false
   slice: interface 3 3
   pointer: true