Separate large perf benchmarks into their own legs (#15231)

Separate large perf benchmarks into their own legs (#15231)

Separate large perf benchmarks into their own legs

This change splits the windows perf test stages into 6 pipelined legs per flavor to reduce the amount of time we spend running the perf tests and reduce the total time of the job. This change also decreases the size of the stashed bin directory by deleting the obj directory. Finally, we move the benchstones suite into one directory (moving BenchF and BenchI into a shared dir called Benchstones)