[X86][AVX512][Clang][Intrinsics][reduce] Adding missing reduce (max|min) intrinsics to Clang .

After LGTM and Check-all 

Vector-reduction arithmetic accepts vectors as inputs and produces 
scalars as outputs.This class of vector operation forms the basis 
of many scientific computations. In vector-reduction arithmetic, 
the evaluation off is independent of the order of the input elements of V.

Reviewer: 1. craig.topper 
          2. igorb

Differential Revision: https://reviews.llvm.org/D25988

llvm-svn: 285493