cmd/compile: -d=checkptr doesn't handle reflect.{Slice,String}Header.Data

This program is valid, yet `-d=checkptr` flags it as unsafe pointer arithmetic:

```
package main

import (
	"reflect"
	"unsafe"
)

var s []int

func main() {
	s = make([]int, 10)
	h := (*reflect.SliceHeader)(unsafe.Pointer(&s))
	println(unsafe.Pointer(h.Data))
}
```

This is because I forgot to implement logic to handle rule 6 for -d=checkptr.