Replaces the DMB ISH + LD + DMB ISHLD sequence with LDAR for volatile field loads - for example, AtomicLong::get.

This is valid, as originally the DMBs were necessary due to the case described here - https://bugs.openjdk.org/browse/JDK-8179954. As in the rare case where the LD can be reordered with an LDAR or STLR from the C2 implementation for stores and loads, these DMBs are required. However, acquire/release operations use a sequentially consistent model which does not allow reordering between them. Hence, the LD can be replaced with an LDAR to disallow reordering with a STLR/LDAR and the first DMB can be removed.

The LDAR has acquire semantics, so it's impossible for memory accesses after to be reordered before; the DMB ISHLD is not required. Therefore, a singular LDAR is sufficient.

This excludes floats and doubles, as they do not have equivalent load-acquire instructions.