Opt Compiler: ARM64 goodness

This patch:
* Switches on PreferAcquireRelease() (used to decide if load/store
volatile should use acquire release-semantics or explicit memory
barriers). Note that for ARMv8 CPUs we should always prefer this
(as proved by synthetic benchmarks on A53, A57 and Denver).

* Enables the use of constants for HBoundsCheck

Change-Id: I42524451772c05a1c74af73e97a59a95f49ba6d4
Signed-off-by: Serban Constantinescu <serban.constantinescu@arm.com>
4 files changed