[InstCombine] canonicalize* LE/GE vector integer comparisons to LT/GT (PR26701, PR26819)

*We don't currently handle the  edge case constants (min/max values), so it's not a complete
canonicalization.

To fully solve the motivating bugs, we need to enhance this to recognize a zero vector
too because that's a ConstantAggregateZero which is a ConstantData, not a ConstantVector
or a ConstantDataVector.

Differential Revision: http://reviews.llvm.org/D17859 

llvm-svn: 269426