ART: Inline MemoryRegion operations

Functions of the MemoryRegion class method are sufficiently simple
yet do not get inlined. Since these methods are called frequently
during stack map generation, inlining them can save a non-negligible
amount of compile time. This patch inserts attributes to force this.

Change-Id: I8ab4f480103fd536f61f52b805864d91bf58428e