[diagnose-unreachable] Constant fold simple switch_enum_addr to elimi…

[gottesmm]

…nate more unreachable code.

This patch comes out of my reading some generic code using .none in transparent
functions to conditionally compile out code at -Onone. Sadly, before this the
dead code in question wouldn't be compiled out unless the protocol was
constrained to be a class protocol.

I added a test that validates that this conditional compilation property can be
relied on in -Onone code in both cases.

[gottesmm]

@swift-ci smoke test

[airspeedswift]

@swift-ci please benchmark

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
SubstringFromLongString	7	8	+14.3%	0.88x (?)
SuffixAnyCollection	12	13	+8.3%	0.92x (?)

Code size: -O

Performance: -Osize

Improvement	OLD	NEW	DELTA	RATIO
PrefixArray	14	13	-7.1%	1.08x (?)

Code size: -Osize

Performance: -Onone

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: 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