[vector] RFR (XS): Missing memory barriers in LibraryCallKit::inline_vector_mem_operation()

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/panama/vector/membar/webrev.00/

Memory accesses with nullable base need memory barriers around them.
Otherwise, stale values can be see since C2 converts them to raw pointers.

In my case, it manifested as spurious default values (all-0s) from 
Vector.defaultReinterpret():

     <F> Vector<F> defaultReinterpret(Species<F> s) {
         int blen = Math.max(s.bitSize(), this.species().bitSize()) / 
Byte.SIZE;
         ByteBuffer bb = 
ByteBuffer.allocate(blen).order(ByteOrder.nativeOrder());
         this.intoByteBuffer(bb, 0);
         return s.fromByteBuffer(bb, 0);
     }

Problematic access came from fromByteBuffer() where ByteBuffer.hb can be 
null (for off-heap buffers):

     public ByteMaxVector fromByteBuffer(ByteBuffer bb, int ix) {
         ...
         return VectorIntrinsics.load(ByteMaxVector.class, byte.class, 
LENGTH,
                               U.getReference(bb, BYTE_BUFFER_HB), ...

Best regards,
Vladimir Ivanov