Add a check for tautological bitwise comparisons to -Wtautological-compare.

This catches issues like:

if ((x & 8) == 4) { ... }
if ((x | 4) != 3) { ... }

Patch by Anders Rönnholm!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209221 91177308-0d34-0410-b5e6-96231b3b80d8