Move some of the decision logic for converting an instruction into one that sets

Move some of the decision logic for converting an instruction into one that sets
the 'zero' bit down into the back-end. There are other cases where this logic
isn't sufficient, so they should be handled separately.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113665 91177308-0d34-0410-b5e6-96231b3b80d8