swift234

rework SGF::emitOptionalToOptional to keep loadable SILValue's as 
registers instead of eagerly dumping them in memory and operating on
them by-address.  This avoids a lot of temporaries and traffic to 
manipulate them.

As part of this, add some new SGF::getOptionalNoneValue/getOptionalSomeValue
helper methods for forming loading optional values.

Many thanks to JoeG for helping with the abstraction difference change in
getOptionalSomeValue.



Swift SVN r27537