swift234

Update function sig opts to work with upcoming call graph changes.

Small tweak to function signature optimization so that it filters out
applies that might have multiple targets, as well as applies of partial
applies, which will take some updates to handle.

Resolves rdar://problem/19632244 and rdar://problem/20049782.

I've opened rdar://problem/20306331 to consider handling
apply-of-partial-apply at some point in the future.

Swift SVN r26585