Invert floating point comparisons such that no extra parity check is required.

This comment pretty much explains it already:

        -- We have to worry about unordered operands (eg. comparisons
        -- against NaN).  If the operands are unordered, the comparison
        -- sets the parity flag, carry flag and zero flag.
        -- All comparisons are supposed to return false for unordered
        -- operands except for !=, which returns true.
        --
        -- Optimisation: we don't have to test the parity flag if we
        -- know the test has already excluded the unordered case: eg >
        -- and >= test for a zero carry flag, which can only occur for
        -- ordered operands.
        --
        -- ToDo: by reversing comparisons we could avoid testing the
        -- parity flag in more cases.

This would turn a sequence of

jp foo
jb bar
jmp foo

into:

jge foo
jmp bar