Add unordered float compares #706

Part of #667

E.g. see https://llvm.org/docs/LangRef.html#fcmp-instruction

The ordinary comparisons like ==, <, >=, etc. should be the ordered comparisons.
That is, they should yield false if either operand is a NaN.

The unordered operations are corresponding operations, but where the result is true if either operand is a NaN. These are interesting and useful because they make it easy to push inversions through:

 ! ( a < b)     

is the same as

(  a  unordered>= b)

(I think I got that right)

I'd like to have a standard way to express these unordered comparisons. It's fine if they are builtin functions with long names.