Use COPY_DEREF instead of COPY_UNREF

Use COPY_DEREF instead of COPY_UNREF

This fixes the behavior when the storage location of the fetch is
modified before the operand is dereferenced by the using VM opcode.

Furthermore it elimiates references as a possible return value from
*_R opcodes, which will give us more opportunities for inferences,
in particular in regard to typed properties.