[DAGCombiner] Added SMAX/SMIN/UMAX/UMIN constant folding

We still need to add constant folding of vector comparisons to fold the tests for targets that don't support the respective min/max nodes

I needed to update 2011-12-06-AVXVectorExtractCombine to load a vector instead of using a constant vector to prevent it folding

Differential Revision: http://reviews.llvm.org/D12118

llvm-svn: 245503