swift234

Teach CopyForwarding to handle enum initialization sequences.

This requires a bit of code motion.

e.g.

1. %Tmp = alloc_stack
2. copy_addr %InArg to [initialization] %Tmp
3. DataAddr = init_enum_data_addr %OutArg
4. copy_addr %Tmp#1 to [initialization] %DataAddr

becomes

1. %Tmp = alloc_stack
4. DataAddr = init_enum_data_addr %OutArg
2. copy_addr %InArg to [initialization] %DataAddr

Fixes at least one regression resulting from '++' removal.
See rdar://23874709 [perf] -Onone Execution Time regression of up-to 19%

-Onone results
|.benchmark............|.bestbase.|.bestopt.|..delta.|.%delta.|speedup.|
|.StringWalk...........|....33570.|...20967.|.-12603.|.-37.5%.|..1.60x.|
|.OpenClose............|......446.|.....376.|....-70.|.-15.7%.|..1.19x.|
|.SmallPT..............|....98959.|...83964.|.-14995.|.-15.2%.|..1.18x.|
|.StrToInt.............|....17550.|...16377.|..-1173.|..-6.7%.|..1.07x.|
|.BenchLangCallingCFunc|......453.|.....428.|....-25.|..-5.5%.|..1.06x.|
|.CaptureProp..........|....50758.|...48156.|..-2602.|..-5.1%.|..1.05x.|
|.ProtocolDispatch.....|.....5276.|....5017.|...-259.|..-4.9%.|..1.05x.|
|.Join.................|.....1433.|....1372.|....-61.|..-4.3%.|..1.04x.|