i386.md (zero_extendhisi2): Split into an expander and anonymous pattern.

        * i386.md (zero_extendhisi2): Split into an expander and anonymous
        pattern.  Add new anonymous pattern for use when optimizing for
        size or for the PPro.
        (zero_extendqihi2, zero_extendqisi2): Likewise.

From-SVN: r25650