GCs (like Shenandoah) might emit loads that don't fit the pattern in Assembler::needs_explicit_null_check(). For example, Shenandoah emits loads for offset=-8 to load an object's forwarding pointer. The existing code would emit explicit null-checks for such loads, however we don't want that because we know that the base is not NULL (everything else would be an error). We need an abstraction for that.