[TargetLowering] fix SETCC SETLT folding with FP types

The bug was introduced with:
https://reviews.llvm.org/rL294863

...and manifests as a selection failure in x86, but that's actually
another bug. This fix prevents wrong codegen with -0.0, but in the
more common case when we have NSZ and NNAN (-ffast-math), we should 
still be able to fold this setcc/compare.

llvm-svn: 294924