Dedicate API should be provided for object loading in c2 optimization stage

This is a follow-up of JDK-8260473.

ArrayCopyNode expansion and Vector unbox expansion use the similar logic to create a barrier resolved LoadNode.
It would be better to provide an API for reducing the code redundancy and simplifying the usage of LoadNode creation during c2 optimization stage, just like 'GraphKit::access_load_at'.