[stdlib] Refactor Dictionary.subscript._modify for better layering

[lorentey]

This may also reduce ARC traffic by a little bit.

[lorentey]

@swift-ci benchmark

[lorentey]

@swift-ci test

[lorentey]

cc @airspeedswift

[swift-ci]

Build comment file:

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
Hanoi	3451	3736	+8.3%	0.92x

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Regression
SubstringFromLongString	11	12	+9.1%	0.92x (?)
Improvement
StringWordBuilderReservingCapacity	2300	2141	-6.9%	1.07x (?)

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

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