Fix ForEachLoopUnroll use-after-free miscompile.

[atrick]

This pass generated incorrect borrow scopes:

%stack = alloc_stack
%borrow = begin_borrow %element
store_borrow %borrow to %stack
end_borrow %borrow
try_apply %f(%stack) normal bb1, error bb2
...
destroy_value %element

This was not showing up as a miscompile before because:

• an array holds an extra copy of the unrolled elements, that array is
  now being optimized away completely.

• CopyPropagation now canonicalizes OSSA lifetimes independent of
  unrelated program side effects.

So, since there is no explicit relationship between %borrow and the
OSSA value in %stack, we end up with:

%stack = alloc_stack
%borrow = begin_borrow %element
store_borrow %borrow to %stack
end_borrow %borrow
destroy_value %element
try_apply %f(%stack) normal bb1, error bb2

Fixes rdar://72904101 ([CanonicalOSSA] Fix ForEachLoopUnroll use-after-free miscompile.)

[atrick]

@swift-ci test

[atrick]

@swift-ci benchmark

[meg-gupta]

lgtm

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - cec5513

[swift-ci]

Performance: -O

Improvement	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	6810	4421	-35.1%	1.54x (?)
String.data.Medium	112	100	-10.7%	1.12x (?)

Code size: -O

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
FlattenListLoop	1627	2505	+54.0%	0.65x (?)
FlattenListFlatMap	5608	6628	+18.2%	0.85x (?)
Improvement	OLD	NEW	DELTA	RATIO
DataReplaceMedium	4600	4200	-8.7%	1.10x (?)

Code size: -Osize

Performance: -Onone

Regression	OLD	NEW	DELTA	RATIO
SevenBoom	1761	2052	+16.5%	0.86x (?)

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 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

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@swift-ci smoke test linux

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[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - cec5513

[atrick]

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