Add Builtin.is_same_metatype to SILCombine.

[atrick]

To optimize String decoding.

Part 3/3 of #30729

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@swift-ci test OS X

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@swift-ci smoke test linux

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@swift-ci benchmark

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeStubFromNSDate	6060	6950	+14.7%	0.87x (?)
EqualSubstringSubstring	40	44	+10.0%	0.91x (?)
EqualSubstringSubstringGenericEquatable	40	44	+10.0%	0.91x (?)
EqualSubstringString	40	44	+10.0%	0.91x (?)
EqualStringSubstring	41	45	+9.8%	0.91x (?)
SortStringsUnicode	2890	3135	+8.5%	0.92x
Improvement	OLD	NEW	DELTA	RATIO
DataReplaceMediumBuffer	4700	4300	-8.5%	1.09x (?)
InsertCharacterEndIndexNonASCII	60	56	-6.7%	1.07x (?)

Code size: -O

Regression	OLD	NEW	DELTA	RATIO
Walsh.o	6364	7028	+10.4%	0.91x
Integrate.o	2462	2510	+1.9%	0.98x

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
EqualSubstringString	40	44	+10.0%	0.91x (?)
EqualSubstringSubstring	41	45	+9.8%	0.91x (?)
LessSubstringSubstring	41	45	+9.8%	0.91x
EqualStringSubstring	41	45	+9.8%	0.91x (?)
EqualSubstringSubstringGenericEquatable	41	45	+9.8%	0.91x (?)
LessSubstringSubstringGenericComparable	41	45	+9.8%	0.91x
SortStringsUnicode	2930	3180	+8.5%	0.92x (?)
Improvement	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeStubToNSDateRef	3960	3580	-9.6%	1.11x (?)
InsertCharacterEndIndexNonASCII	60	56	-6.7%	1.07x (?)

Code size: -Osize

Regression	OLD	NEW	DELTA	RATIO
Integrate.o	2478	2542	+2.6%	0.97x
Walsh.o	4322	4426	+2.4%	0.98x

Performance: -Onone

Regression	OLD	NEW	DELTA	RATIO
SortStringsUnicode	4305	4740	+10.1%	0.91x (?)
EqualSubstringSubstringGenericEquatable	47	51	+8.5%	0.92x (?)
LessSubstringSubstringGenericComparable	47	51	+8.5%	0.92x (?)
Improvement	OLD	NEW	DELTA	RATIO
FindString.Rec3.String	983	894	-9.1%	1.10x (?)
FindString.Rec3.Substring	919	842	-8.4%	1.09x (?)

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