Some cleanups while looking at these two functions.

gcc/ChangeLog:

    * config/i386/i386-expand.cc (ix86_expand_vecop_qihi2): Also
    reject ymm instructions for TARGET_PREFER_AVX128.  Use generic
    gen_extend_insn to generate zero/sign extension instructions.
    Fix comments.
    (ix86_expand_vecop_qihi): Initialize interleave functions
    for MULT code only.  Fix comments.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

Pushed to master.

Uros.