Improve optimizer fast path to not even run optimize_mir_local

[ggevay]

#28715 introduced an optimizer fast path, which avoids running a large part of the optimizer pipeline if a query looks like it will be a fast path peek. However, we still run optimize_mir_local, which I think we could also avoid (with a somewhat larger refactoring than #28715)

Currently, optimization time is only a small portion of fast path peek time, but if this becomes significant again after optimizations in other parts of the system, then we should revisit this.