Improve implementation of bidi-value()

Before these changes the following input:

    p {
        bidi-value(float, left, right);
    }

... would compile to the following output:

    p {
        float: left;
    }

    .html-rtl p {
        float: left;
        float: right;
    }

After these changes, the unnecessary "float: left" in the RTL version is
not included. This saves about 250 bytes of compiled CSS -- it's
something!