give instcombine some helper functions for matching MIN and MAX, and

give instcombine some helper functions for matching MIN and MAX, and
implement some optimizations for MIN(MIN()) and MAX(MAX()) and
MIN(MAX()) etc.  This substantially improves the code in PR5822 but
doesn't kick in much elsewhere.  2 max's were optimized in
pairlocalalign and one in smg2000.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@91814 91177308-0d34-0410-b5e6-96231b3b80d8