i386: Cleanup ix86_expand_vecop_qihi{,2}

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.