Fix SimplifyCFG CheckedCastBrJumpThreading.

[atrick]

This pass was rewriting branches using the orignal branch target
instead of the new branch target. It used to not matter when the pass
was mannually splitting critical edges later. Now the splitting
is handled automatically.

Fixes rdar://71447520 (SILVerifier error after SimplifyCFG
"Instruction does not dominate all uses!")

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[atrick]

@swift-ci test

[atrick]

@swift-ci test source compatibility

[atrick]

@swift-ci benchmark

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
FlattenListLoop	1628	2503	+53.7%	0.65x (?)
FlattenListFlatMap	4931	6798	+37.9%	0.73x (?)
Improvement	OLD	NEW	DELTA	RATIO
NSStringConversion.UTF8	1020	917	-10.1%	1.11x (?)

Code size: -O

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	3887	4822	+24.1%	0.81x (?)
PrefixWhileAnySequenceLazy	1634	1913	+17.1%	0.85x (?)
Improvement	OLD	NEW	DELTA	RATIO
String.data.LargeUnicode	124	114	-8.1%	1.09x (?)
ObjectiveCBridgeFromNSDictionaryAnyObjectForced	9700	9050	-6.7%	1.07x (?)

Code size: -Osize

Performance: -Onone

Regression	OLD	NEW	DELTA	RATIO
DataAppendBytesMedium	6080	6880	+13.2%	0.88x (?)
UTF8Decode_InitFromBytes_ascii_as_ascii	490	550	+12.2%	0.89x (?)

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

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 71acf8d