[DAGCombiner] Fix PR25763 - vector comparison constant folding + sign-extension

PR25763 demonstrated an issue with D14683 - vector comparison constant folding only works for i1 results, so we need to split off the sign-extension of the result to the required type. Luckily this can be done with the existing type legalization code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255289 91177308-0d34-0410-b5e6-96231b3b80d8