Devirtualizer: fix a miscompile due to handling of cast instructions.

[eeckstein]

getInstanceWithExactDynamicType returns a new instance, and for this the class decl has to be updated.

https://bugs.swift.org/browse/SR-12538
rdar://problem/61911112

[eeckstein]

@swift-ci test

[eeckstein]

@swift-ci benchmark

[swift-ci]

Performance: -O

Code size: -O

Performance: -Osize

Improvement	OLD	NEW	DELTA	RATIO
FlattenListLoop	3037	2733	-10.0%	1.11x (?)

Code size: -Osize

Performance: -Onone

Improvement	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeStubNSDateRefAccess	3700	3449	-6.8%	1.07x (?)

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