Mark all PPC comparison instructions as not having side effects

Now that the CR spilling issues have been resolved, we can remove the
unmodeled-side-effect attributes from the comparison instructions (and also
mark them as isCompare). By allowing these, by default, to have unmodeled side
effects, we were hiding problems with CR spilling; but everything seems much
happier now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179502 91177308-0d34-0410-b5e6-96231b3b80d8