benchmarks: Fix an integer overflow problem in the MapReduce benchmarks on 32-bit targets

[eeckstein]

No description provided.

[eeckstein]

@swift-ci benchmark

[eeckstein]

@swift-ci smoke test

[swift-ci]

Performance (x86_64): -O

Regression	OLD	NEW	DELTA	RATIO
MapReduceAnyCollection	87	124	+42.5%	0.70x

Code size: -O

Performance (x86_64): -Osize

Regression	OLD	NEW	DELTA	RATIO
MapReduceLazyCollectionShort	32	47	+46.9%	0.68x
FlattenListLoop	956	1404	+46.9%	0.68x (?)
FlattenListFlatMap	2630	3155	+20.0%	0.83x (?)
Improvement	OLD	NEW	DELTA	RATIO
ArrayAppendGenericStructs	1550	590	-61.9%	2.63x (?)
MapReduceString	44	41	-6.8%	1.07x (?)

Code size: -Osize

Performance (x86_64): -Onone

Improvement	OLD	NEW	DELTA	RATIO
ArrayAppendGenericStructs	1530	1250	-18.3%	1.22x (?)
NSError	472	437	-7.4%	1.08x (?)

Code size: -swiftlibs

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: 6-Core 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