[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

llvm-svn: 332031