LoopVectorize: We don't need an identity element for min/max reductions

LoopVectorize: We don't need an identity element for min/max reductions

We can just use the initial element that feeds the reduction.

  max(max(x, y), z) == max(max(x,y), max(x,z))

radar://13723044

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181141 91177308-0d34-0410-b5e6-96231b3b80d8