Fix postinc/dec operations on accessors.

The code I wrote for postinc/dec on setters turned out to be bogus. This should fix it.

Normally you're supposed to use `check_userop` to do the heavy lifting for inc/dec operations. Unfortunately it's a big hack (surprise). spcomp fetches the value twice, which in this case, would involve calling the getter twice.

The new code changes `check_userop` to treat accessors as if they were rvalues - so it doesn't try to save/restore anything. The code before `check_userop` sets up the stack beforehand so we can safely recover the original object and pre-incremented value later.

In the vast majority of cases we won't have a user operator, so all the saving/restoring work is pointless. But like most of spcomp it's pretty hard to disentangle semantic logic from code generation.