propagate 'unpredictable' metadata on select instructions

This is similar to D18133 where we allowed profile weights on select instructions. 
This extends that change to also allow the 'unpredictable' attribute of branches to apply to selects.

A test to check that 'unpredictable' metadata is preserved when cloning instructions was checked in at:
http://reviews.llvm.org/rL263648

Differential Revision: http://reviews.llvm.org/D18220

llvm-svn: 263716