Remove implicit `= noreg` temporary register arguments for the three methods that still have them.
  * `load_heap_oop`
  * `store_heap_oop`
  * `load_heap_oop_not_null`

Only `load_heap_oop` is used with the implicit `= noreg` arguments.
After JDK-8293769, the GCs only use explicitly passed in registers. This will also be the case for generational ZGC.
Where it currently requires `load_heap_oop` to provide a second temporary register.