CSE/OSSA avoid endless cloning and reoptimization

[atrick]

Prevent CSE from introducing useless copies, borrows, and
clones. Otherwise it will endlessly clone and re-cse the same
projections endlessly.

TODO: Most of these cases can be handled GuarateedOwnershipExtension
or extendOwnedLifetime without requiring any copies!

[atrick]

@swift-ci test

[atrick]

@swift-ci benchmark

[meg-gupta]

thanks!

[swift-ci]

Performance (x86_64): -O

Regression	OLD	NEW	DELTA	RATIO
ArrayAppendOptionals	610	1040	+70.5%	0.59x (?)
StringBuilder	189	210	+11.1%	0.90x (?)
StringUTF16Builder	200	220	+10.0%	0.91x (?)
StringBuilderSmallReservingCapacity	201	218	+8.5%	0.92x (?)

Code size: -O

Performance (x86_64): -Osize

Regression	OLD	NEW	DELTA	RATIO
StringBuilder	190	216	+13.7%	0.88x (?)
StringBuilderSmallReservingCapacity	199	225	+13.1%	0.88x (?)
Improvement	OLD	NEW	DELTA	RATIO
FlattenListLoop	1571	1320	-16.0%	1.19x (?)

Code size: -Osize

Performance (x86_64): -Onone

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