Fix SR-4601 Report Added and Removed Benchmarks in Performance Comparison (swiftlang#8991)

* Refactor compare_perf_tests.py

* Fix SR-4601 Report Added and Removed Benchmarks in Performance Comparison

Improved HTML styling.

* Added back support for reading concatenated Benchmark_Driver output

PerformanceTestResults can be merged, computing new MIN, MAX, and running MEAN and SD.

* Handle output from Benchmark_O again

Treat MAX_RSS as optional column

Author: palimondo