[InstCombine] check vector elements before trying to transform LE/GE vector icmp (PR27756)

Fix a bug introduced with rL269426 :
[InstCombine] canonicalize* LE/GE vector integer comparisons to LT/GT (PR26701, PR26819)

We were assuming that a ConstantDataVector / ConstantVector / ConstantAggregateZero operand of
an ICMP was composed of ConstantInt elements, but it might have ConstantExpr or UndefValue 
elements. Handle those appropriately.

Also, refactor this function to join the scalar and vector paths and eliminate the switches.

Differential Revision: http://reviews.llvm.org/D20289

llvm-svn: 269728