Adjust invocation count during warmup stage

Suggested by @kouvel (see https://github.com/dotnet/coreclr/issues/25680#issuecomment-514024669):
> The pilot phase runs early before warmup and perf could be very different in some cases during the pilot phase and after warmup when tiering is enabled. If piloting completes before the main parts of the benchmark are tiered up, the invocation count that is determined could be much lower compared with tiering disabled. Some benchmarks may perform differently at different invocation counts. Perhaps the warmup phase could also adjust invocation counts, and overhead could be measured after warmup (if overhead measurement is dependent on the invocation count)?