swift234

rework PatternMatchEmission::emitSharedCaseBlocks to avoid emitting empty blocks 
(containing just an uncond branch) when a shared block isn't actually shared.

This is a revised version of r26676 that makes sure to emit the cleanups for a
case pattern as soon as possible.  Extending the lifetime of the case value
across the body of the switch caused extra copies of COW types.


Swift SVN r26685