[InstCombine] only allow narrow/wide evaluation of values with >1 use if that user is a binop

There was a logic hole in D42739 / rL324014 because we're not accounting for select and phi
instructions that might have repeated operands. This is likely a source of an infinite loop.
I haven't manufactured a test case to prove that, but it should be safe to speculatively limit
this transform to binops while we try to create that test.

llvm-svn: 324252