Nashorn needs to reuse temporary symbols

Profiling with --print-mem-usage indicates that symbols are a heavy part of our IR. Running a large script like Mandreel gives us ~ 300000 symbols where the majority are temporaries. Temporaries are only live in one expression tree at a time, so it should be perfectly possible to reuse them (if previous expression trees had created symbols with the type we need for a new temporary) rather than allocating new temporaries every time we need them. This will have immense trickle down effects for our memory usage and also speed up symbol lookups significantly - we do a lot of block.getExistingSymbol in large linked hash maps with plenty of chaining.