Optimizing: Tag arena allocations in code generators.

And completely remove the deprecated GrowableArray.

Replace GrowableArray with ArenaVector in code generators
and related classes and tag arena allocations.

Label arrays use direct allocations from ArenaAllocator
because Label is non-copyable and non-movable and as such
cannot be really held in a container. The GrowableArray
never actually constructed them, instead relying on the
zero-initialized storage from the arena allocator to be
correct. We now actually construct the labels.

Also avoid StackMapStream::ComputeDexRegisterMapSize() being
passed null references, even though unused.

Change-Id: I26a46fdd406b23a3969300a67739d55528df8bf4