Add init_existential_metatype to CSE.

[atrick]

So String decoding can be optimized.

Part 2/3 of optimizations for #30729

[atrick]

@swift-ci test

[atrick]

@swift-ci benchmark

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
ArrayLiteral2	67	79	+17.9%	0.85x (?)
Improvement	OLD	NEW	DELTA	RATIO
ParseFloat.Float.Exp	10	9	-10.0%	1.11x (?)

Code size: -O

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
String.data.Empty	24	26	+8.3%	0.92x (?)
SubstringFromLongStringGeneric	12	13	+8.3%	0.92x (?)
Set.isStrictSubset.Int.Empty	49	53	+8.2%	0.92x (?)
String.data.Small	25	27	+8.0%	0.93x (?)
Set.subtracting.Empty.Int	26	28	+7.7%	0.93x (?)
Improvement	OLD	NEW	DELTA	RATIO
PrefixWhileAnySeqCRangeIter	213	199	-6.6%	1.07x (?)

Code size: -Osize

Performance: -Onone

Regression	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeStubNSDateRefAccess	3735	4092	+9.6%	0.91x (?)
Improvement	OLD	NEW	DELTA	RATIO
ArrayAppendLatin1Substring	34236	30888	-9.8%	1.11x (?)
ArrayAppendUTF16Substring	33804	30564	-9.6%	1.11x (?)
Breadcrumbs.MutatedUTF16ToIdx.ASCII	15	14	-6.7%	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

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - a22d0f2

[atrick]

@swift-ci test linux

[atrick]

Merging so I can post part 3/3 of this fix