[InstCombine] Remove check for sext of vector icmp from shouldOptimizeCast

Looks like for 'and' and 'or' we end up performing at least some of the transformations this is bocking in a round about way anyway.

For 'and sext(cmp1), sext(cmp2) we end up later turning it into 'select cmp1, sext(cmp2), 0'. Then we optimize that back to sext (and cmp1, cmp2). This is the same result we would have gotten if shouldOptimizeCast hadn't blocked it. We do something analogous for 'or'.

With this patch we allow that transformation to happen directly in foldCastedBitwiseLogic. And we now support the same thing for 'xor'. This is definitely opening up many other cases, but since we already went around it for some cases hopefully it's ok.

Differential Revision: https://reviews.llvm.org/D36213

llvm-svn: 311508