Update some parts of SimplifyCFG for OSSA

[meg-gupta]

This PR fixes and adds OSSA tests for the following SimplifyCFG transformations:

SimplifyCFG::simplifySwitchEnumBlock
SimplifyCFG::simplifySwitchEnumUnreachableBlocks
SimplifyCFG::simplifyTermWithIdenticalDestBlocks
SimplifyCFG::simplifyArgs
SimplifyCFG::simplifySwitchEnumOnObjcClassOptional

Left over transformations in SimplifyCFG not included in this PR are jump threading, dominator based simplifications and argument splitting, which will be posted in a subsequent PR.

[meg-gupta]

@swift-ci test

[meg-gupta]

@swift-ci test source compatibility

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@swift-ci benchmark

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@swift-ci test

[meg-gupta]

@swift-ci test source compatibility

[meg-gupta]

@swift-ci benchmark

[meg-gupta]

------- Performance (x86_64): -O -------

REGRESSION OLD NEW DELTA RATIO
Data.append.Sequence.64kB.Count 2.0 2.667 +33.3% 0.75x (?)
EqualStringSubstring 25.828 30.913 +19.7% 0.84x (?)
StringFromLongWholeSubstringGeneric 4.783 5.685 +18.9% 0.84x (?)
EqualSubstringSubstringGenericEquatable 26.063 30.6 +17.4% 0.85x (?)
LessSubstringSubstringGenericComparable 26.125 30.588 +17.1% 0.85x (?)
EqualSubstringString 26.412 30.8 +16.6% 0.86x (?)
LessSubstringSubstring 26.283 30.594 +16.4% 0.86x (?)
EqualSubstringSubstring 26.71 30.92 +15.8% 0.86x
DictionaryKeysContainsCocoa 14.537 16.452 +13.2% 0.88x (?)
StringComparison_longSharedPrefix 305.857 341.0 +11.5% 0.90x (?)
DistinctClassFieldAccesses 40.378 44.735 +10.8% 0.90x (?)
StringSwitch 224.5 243.143 +8.3% 0.92x (?)

IMPROVEMENT OLD NEW DELTA RATIO
ArrayAppendGenericStructs 1520.0 616.667 -59.4% 2.46x (?)
FlattenListLoop 1616.0 997.5 -38.3% 1.62x (?)
FlattenListFlatMap 3963.0 2875.0 -27.5% 1.38x (?)
String.replaceSubrange.Substring.Small 50.2 39.409 -21.5% 1.27x (?)
StringUTF16SubstringBuilder 1626.364 1413.0 -13.1% 1.15x (?)
StringComparison_ascii 368.0 328.5 -10.7% 1.12x (?)
OpenClose 54.435 50.08 -8.0% 1.09x (?)
String.replaceSubrange.ArrChar.Small 40.273 37.625 -6.6% 1.07x (?)

------- Performance (x86_64): -Osize -------

REGRESSION OLD NEW DELTA RATIO
EqualSubstringSubstring 25.304 30.586 +20.9% 0.83x (?)
StringFromLongWholeSubstringGeneric 4.717 5.64 +19.6% 0.84x (?)
EqualStringSubstring 25.852 30.577 +18.3% 0.85x (?)
EqualSubstringSubstringGenericEquatable 25.912 30.594 +18.1% 0.85x (?)
EqualSubstringString 25.963 30.594 +17.8% 0.85x (?)
PrefixWhileAnySequence 191.2 224.5 +17.4% 0.85x (?)
LessSubstringSubstringGenericComparable 26.059 30.594 +17.4% 0.85x (?)
LessSubstringSubstring 26.061 30.594 +17.4% 0.85x (?)
DistinctClassFieldAccesses 37.128 41.467 +11.7% 0.90x (?)
StringComparison_longSharedPrefix 306.429 342.0 +11.6% 0.90x (?)
Set.isStrictSubset.Int.Empty 39.208 43.561 +11.1% 0.90x (?)
DictionaryKeysContainsCocoa 14.324 15.741 +9.9% 0.91x (?)
PrefixWhileSequence 189.6 205.667 +8.5% 0.92x (?)
StringHasSuffixUnicode 69350.0 75157.895 +8.4% 0.92x (?)

IMPROVEMENT OLD NEW DELTA RATIO
String.replaceSubrange.Substring.Small 49.957 39.519 -20.9% 1.26x (?)
StringUTF16SubstringBuilder 1871.111 1667.778 -10.9% 1.12x (?)
Chars2 3814.063 3487.143 -8.6% 1.09x (?)
InsertCharacterTowardsEndIndex 146.417 134.727 -8.0% 1.09x (?)
StringAdder 287.625 268.333 -6.7% 1.07x (?)

------- Performance (x86_64): -Onone -------

REGRESSION OLD NEW DELTA RATIO
StringFromLongWholeSubstringGeneric 15.652 21.71 +38.7% 0.72x (?)
LessSubstringSubstringGenericComparable 27.778 33.652 +21.1% 0.83x (?)
EqualStringSubstring 30.172 35.381 +17.3% 0.85x (?)
EqualSubstringString 30.867 35.563 +15.2% 0.87x (?)
EqualSubstringSubstringGenericEquatable 29.178 33.533 +14.9% 0.87x (?)
EqualSubstringSubstring 30.645 35.083 +14.5% 0.87x (?)
LessSubstringSubstring 30.839 34.971 +13.4% 0.88x (?)
SetSymmetricDifferenceInt100 405.6 441.4 +8.8% 0.92x (?)

IMPROVEMENT OLD NEW DELTA RATIO
String.replaceSubrange.Substring.Small 51.727 41.167 -20.4% 1.26x (?)
InsertCharacterTowardsEndIndex 162.545 150.583 -7.4% 1.08x (?)

[meg-gupta]

@swift-ci test windows platform

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@swift-ci benchmark

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@swift-ci test

[atrick]

Just a couple of comments... Looks great!

[meg-gupta]

@swift-ci test

[meg-gupta]

@swift-ci test

[meg-gupta]

@swift-ci smoke test and merge