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.
AfterJDK-8293351 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.
* `load_heap_oop`
* `store_heap_oop`
* `load_heap_oop_not_null`
Only `load_heap_oop` is used with the implicit `= noreg` arguments.
After