[gardening] Clean up many String computed vars

[milseman]

No description provided.

[milseman]

@swift-ci please test

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[milseman]

@swift-ci please test

[milseman]

@swift-ci please benchmark

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Build failed
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Git Sha - 84e3a1e4505748e4e9b99ef49be889257314fc84

[milseman]

@swift-ci please test

[milseman]

@swift-ci please benchmark

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Build failed
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Git Sha - 416b1c7b1ce7f818c60c36e4c11224b6f360b4f4

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[swift-ci]

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
EqualStringSubstring	42	38	-9.5%	1.11x (?)
EqualSubstringString	42	38	-9.5%	1.11x
LessSubstringSubstring	43	39	-9.3%	1.10x
EqualSubstringSubstring	41	38	-7.3%	1.08x (?)
EqualSubstringSubstringGenericEquatable	41	38	-7.3%	1.08x (?)
LessSubstringSubstringGenericComparable	42	39	-7.1%	1.08x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Improvement
LessSubstringSubstring	43	39	-9.3%	1.10x (?)
EqualSubstringString	43	39	-9.3%	1.10x (?)
LessSubstringSubstringGenericComparable	43	39	-9.3%	1.10x (?)
EqualSubstringSubstring	42	39	-7.1%	1.08x (?)
EqualStringSubstring	42	39	-7.1%	1.08x (?)

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Improvement
EqualSubstringSubstringGenericEquatable	48	44	-8.3%	1.09x (?)
LessSubstringSubstringGenericComparable	48	44	-8.3%	1.09x

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 Pro
  Model Identifier: MacPro6,1
  Processor Name: 12-Core Intel Xeon E5
  Processor Speed: 2.7 GHz
  Number of Processors: 1
  Total Number of Cores: 12
  L2 Cache (per Core): 256 KB
  L3 Cache: 30 MB
  Memory: 64 GB

[milseman]

@swift-ci please test

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Build failed
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Git Sha - c71343371adc259caa90baaa0ea7a590b9a338f5

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Build failed
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Git Sha - c71343371adc259caa90baaa0ea7a590b9a338f5

[milseman]

Expr/TestIRMemoryMap.test

[milseman]

@swift-ci please test Linux platform