[InstCombine] handle zero constant vectors for LE/GE comparisons too

Enhancement to: http://reviews.llvm.org/rL269426
With discussion in: http://reviews.llvm.org/D17859

This should complete the fixes for: PR26701, PR26819:
https://llvm.org/bugs/show_bug.cgi?id=26701
https://llvm.org/bugs/show_bug.cgi?id=26819
 

llvm-svn: 269439