[analyzer] Don't emit an "Assuming x is <OP> y" if it's not a comparison op.

We could certainly be more precise in many of our diagnostics, but before we
were printing "Assuming x is && y", which is just ridiculous.

<rdar://problem/15167979>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193455 91177308-0d34-0410-b5e6-96231b3b80d8