Avoiding multiple cache insertions in ValuePlug::Computation::compute()

Previously, it was quite common for a given computation result to get inserted into the cache multiple times, resulting in more calls to memoryUsage() than necessary and a very significant performance penalty. I'm now skipping the insertion if the value is already in the cache, resulting in an 85% improvement in startup time for one of my test renders.
Update comment in ValuePlug.cpp.