SI-7458 Pres. compiler must not observe trees in silent mode

Otherwise we can think that `+` in `1 + BigInt(2)` refers
to a method in `Int`.

In general, this protects the IDE from observing results from
"exploratory" typing which is discarded as the compiler backtracks
to another possibility.

This protection subsumes the condition that checked for overloaded
types: presentation/t7458 now passes without this.