swift234

teach silcombine to merge upcast instructions that feed each other, now that
sema is emitting each step of a conversion independently.  Also, as a driveby,
fix an optimizer crash in the "(ref_to_raw_pointer (ref_to_object_pointer x)) 
to (ref_to_raw_pointer x)" peephole when the ref_to_object_pointer had other
uses (noticed by inspection).



Swift SVN r14603