[benchmark] BucketSort gardening and adjustments

[palimondo]

Increased workload to sorting of 10k items.
Used SplitMix64 PRNG for randomness.
Adjusted formatting and naming to match Swift conventions.
Adjusted documentation.

[palimondo]

@rajbarik, @atrick I took a swing at adjusting the BucketSort benchmark along the comments in #24206. Please review.

[palimondo]

@swift-ci please benchmark

[palimondo]

@swift-ci please smoke test

[swift-ci]

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
BucketSort	24940	2539	-89.8%	9.82x
ObjectiveCBridgeStubToNSStringRef	80	73	-8.7%	1.10x (?)

Code size: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
BucketSort.o	9400	11056	+17.6%	0.85x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
StringBuilderWithLongSubstring	950	1070	+12.6%	0.89x (?)
Improvement
BucketSort	24620	2536	-89.7%	9.71x
SuffixCountableRangeLazy	5	4	-20.0%	1.25x (?)

Code size: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
BucketSort.o	9402	11162	+18.7%	0.84x

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Improvement
BucketSort	40570	8554	-78.9%	4.74x

How to read the data

The tables contain differences in performance which are larger than 8% and differences in code size which are larger than 1%.

If you see any unexpected regressions, you should consider fixing the
regressions before you merge the PR.

Noise: Sometimes the performance results (not code size!) contain false
alarms. Unexpected regressions which are marked with '(?)' are probably noise.
If you see regressions which you cannot explain you can try to run the
benchmarks again. If regressions still show up, please consult with the
performance team (@eeckstein).

Hardware Overview

  Model Name: Mac mini
  Model Identifier: Macmini8,1
  Processor Name: Intel Core i7
  Processor Speed: 3.2 GHz
  Number of Processors: 1
  Total Number of Cores: 6
  L2 Cache (per Core): 256 KB
  L3 Cache: 12 MB
  Memory: 64 GB

[palimondo]

Note: the reported performance change is caused by the removal of legacyFactor.

[atrick]

Thanks.

[atrick]

Merging now since this changes legacyFactor to have a stable baseline before @rajbarik enables Existential Specialization.