Dump scheduling statistics for collecting dispatch numbers (#12757)

Use option `--iree-scheduling-dump-statistics-format` to dump scheduling
statistics at stream level, which contains the number of dispatches,
when compiling the benchmark suites.

https://github.com/openxla/iree/blob/97779d7f494660f88864b035475ec77a1e54c6c8/compiler/src/iree/compiler/Dialect/Stream/Transforms/Passes.cpp#L312

This allows us to collect and track the changes in the number of
dispatches, which is a requested feature for a long time. It seems fine
to upload this metric to https://perf.iree.dev, as now we use filters to
better organize the metrics (and the dashboard performance looks fine).
I still want to have a better way to structure the metrics, with the
database later...

The benchmark tool will be updated later to gather these information.

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Co-authored-by: Geoffrey Martin-Noble <gcmn@google.com>