[InstCombine] reorder folds to reduce chance of infinite loops

I don't have a test case for this, but it's motivated by
the discussion in D51964, and I've added TODO comments for
the better fix - move simplifications into instsimplify
because that's more efficient and reduces risk of infinite
loops in instcombine caused by transforms trying to do the
opposite folds.

In this case, we know that the transform that tries to move
'not' through min/max can be fooled by the multiple uses
of a value in another min/max, so try to squash the 
foldSPFofSPF() patterns first.

llvm-svn: 342147