SILCombine: fix a miscompile in the alloc_stack optimization which causes a use-after-free.

[eeckstein]

A "copy_addr [take] %src to [initialization] %alloc_stack" is replaced by a "destroy_addr %src" if the alloc_stack is otherwise dead.
This is okay as long as the "moved" object is kept alive otherwise.
This can break if a retain of %src is moved after the copy_addr. It cannot happen with OSSA.
So as soon as we have OSSA, we can remove the check again.

rdar://problem/59496027

[eeckstein]

@swift-ci test

[eeckstein]

@swift-ci benchmark

[swift-ci]

Performance: -O

Improvement	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeFromNSArrayAnyObjectForced	5340	4920	-7.9%	1.09x (?)

Code size: -O

Performance: -Osize

Code size: -Osize

Performance: -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 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

[aschwaighofer]

lgtm.