By far, one of the most common uses of isnan is to make 'isunordered'

By far, one of the most common uses of isnan is to make 'isunordered'
comparisons.  In an 'isunordered' predicate, which looks like this at
the LLVM level:

        %a = call bool %llvm.isnan(double %X)
        %b = call bool %llvm.isnan(double %Y)
        %COM = or bool %a, %b

We used to generate this code:

        fxch %ST(1)
        fucomip %ST(0), %ST(0)
        setp %AL
        fucomip %ST(0), %ST(0)
        setp %AH
        or %AL, %AH

With this patch, we generate this code:

        fucomip %ST(0), %ST(1)
        fstp %ST(0)
        setp %AL

Which should make alkis happy.  Tested as X86/compare_folding.llx:test1

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@14148 91177308-0d34-0410-b5e6-96231b3b80d8