[DAGCombiner] Vector constant folding for comparisons

[DAGCombiner] Vector constant folding for comparisons

This patch adds support for vector constant folding of integer/float comparisons.

This requires FoldConstantVectorArithmetic to support scalar constant operands (in this case ISD::CONDCASE). In future we should be able to support other scalar constant types as necessary (and possibly start calling FoldConstantVectorArithmetic for all node creations)

Differential Revision: http://reviews.llvm.org/D14683

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253504 91177308-0d34-0410-b5e6-96231b3b80d8