SILGen: Don't reabstract a member ref we're about to directly apply.

[jckarter]

SILGenApply can work with a function value at any abstraction level, so if we're loading a member out of an
aggregate that's stored at a different abstraction level than the substituted abstraction level, we can
just apply it the way it is without reabstracting.

[jckarter]

@swift-ci Please test

[jckarter]

@swift-ci Please benchmark

[swift-ci]

Performance (x86_64): -O

Regression	OLD	NEW	DELTA	RATIO
DictionaryOfAnyHashableStrings_insert	3178	5558	+74.9%	0.57x
Set.isDisjoint.Box25	356	514	+44.4%	0.69x (?)
Set.isDisjoint.Int50	268	340	+26.9%	0.79x (?)
UTF8Decode_InitFromBytes_ascii_as_ascii	483	557	+15.3%	0.87x (?)
NSError	157	181	+15.3%	0.87x (?)
Improvement	OLD	NEW	DELTA	RATIO
FlattenListLoop	2492	1628	-34.7%	1.53x (?)
FlattenListFlatMap	6463	4363	-32.5%	1.48x (?)

Code size: -O

Performance (x86_64): -Osize

Regression	OLD	NEW	DELTA	RATIO
DictionaryKeysContainsCocoa	26	31	+19.2%	0.84x (?)
FlattenListFlatMap	3965	4299	+8.4%	0.92x (?)

Code size: -Osize

Performance (x86_64): -Onone

Regression	OLD	NEW	DELTA	RATIO
StringToDataMedium	4450	4950	+11.2%	0.90x (?)
DataAppendBytesMedium	4700	5180	+10.2%	0.91x (?)

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