Assign-ops and incdec on overloaded properties are implemented
using a read_property followed by write_property. Previously, if
__get() returned by-reference, pre-incdec and assign-op
additionally also modified the reference, while post-incdec worked
correctly.
This change synchronizes the three code-paths to not modify the
reference. The pre-incdec implementation matches the post-incdec
implementation, the assign-op implementation uses a distinct
result operand.
