Generate floating-point comparisons on x86 which deal with NaNs in what
I assume is an IEEE854 compliant fashion.  For
   == >= > <= <
if either arg is a NaN, produce False, and for
   /=
if either arg is a NaN, produce True.

This is the behaviour that gcc has, by default.

Requires some ultramagical x86 code frags to be emitted.  A big comment
in PprMach explains how it works.