[InstCombine] don't form select from bitcasted logic ops if bitcasts have >1 use

This isn't a sure thing (are 2 extra bitcasts less expensive than a logic op?), 
but we'll try to err on the conservative side by going with the case that has
less IR instructions.

Note: This question came up in http://reviews.llvm.org/D22114 , but this part is
independent of that patch proposal, so I'm making this small change ahead of that
one. 

See also:
http://reviews.llvm.org/rL274926

llvm-svn: 274932