[InstCombine] add folds for minnum(-a, -b) --> -maxnum(a, b)

This is similar to what we do for integer min/max with 'not'
ops (rL321882).

This should fix:
https://bugs.llvm.org/show_bug.cgi?id=37404
https://bugs.llvm.org/show_bug.cgi?id=37405


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332031 91177308-0d34-0410-b5e6-96231b3b80d8