SILOptimizer: fix a phase ordering problem, which prevented array optimizations to work in some cases

[eeckstein]

Introduce an "early redundant load elimination", which does not optimize loads from arrays.
Later array optimizations, like ABCOpt, get confused if an array load in a loop is converted to a pattern with a phi argument.

This problem was introduced with accessors.

rdar://problem/44184763

[eeckstein]

@swift-ci test

[eeckstein]

@swift-ci benchmark

[swift-ci]

Build comment file:

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
XorLoop	555	396	-28.6%	1.40x
RemoveWhereSwapInts	25	19	-24.0%	1.32x
RemoveWhereMoveInts	14	12	-14.3%	1.17x
ObjectiveCBridgeStubToNSDate2	1471	1336	-9.2%	1.10x (?)

Code size: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
Queue.o	12219	14587	+19.4%	0.84x
Improvement
Prims.o	43305	41857	-3.3%	1.03x
PrimsSplit.o	43353	41905	-3.3%	1.03x
RC4.o	5023	4959	-1.3%	1.01x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
IterateData	1594	1808	+13.4%	0.88x (?)
Improvement
XorLoop	613	407	-33.6%	1.51x
RemoveWhereSwapInts	25	19	-24.0%	1.32x
RemoveWhereMoveInts	14	12	-14.3%	1.17x

Code size: -Osize

TEST	OLD	NEW	DELTA	RATIO
Improvement
XorLoop.o	2122	2074	-2.3%	1.02x
RC4.o	4137	4073	-1.5%	1.02x

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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[atrick]

LGTM