[simplify-cfg] Add a visitedBlocks set to hasSameUltimateSuccessor to prevent infinite loops.

[gottesmm]

Previously, we were not handling properly blocks that we could visit multiple
times. In this commit, I added a SmallPtrSet to ensure that we handle all of the
same cases that we handled previously.

The key reason that we want to follow this approach rather than something else
is that the previous algorithm on purpose allowed for side-entrances from other
checks since often times when we have multiple checks, all of the .none branches
funnel together into a single ultimate block.

This can be seen by the need of this code to support the test two_chained_calls
in simplify_switch_enum_objc.sil.

rdar://55861081

[gottesmm]

@swift-ci test

[gottesmm]

@swift-ci benchmark

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
DataAppendDataSmallToSmall	2760	3200	+15.9%	0.86x (?)
Improvement	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	4902	4573	-6.7%	1.07x (?)

Code size: -O

Performance: -Osize

Code size: -Osize

Performance: -Onone

Regression	OLD	NEW	DELTA	RATIO
DataAppendDataSmallToSmall	2820	3240	+14.9%	0.87x (?)

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: 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

[eeckstein]

lgtm!