[DAGCombiner] add fold for 'All sign bits set?'

(and (setlt X,  0), (setlt Y,  0)) --> (setlt (and X, Y),  0)

We have 7 similar folds, but this one got away. The fact that the
x86 test with a branch didn't change is probably a separate bug. We
may also be missing this and the related folds in instcombine.

llvm-svn: 299252