X86: Custom lower zext v16i8 to v16i16.

On sandy bridge (PR17654) we now get
	vpxor	%xmm1, %xmm1, %xmm1
	vpunpckhbw	%xmm1, %xmm0, %xmm2
	vpunpcklbw	%xmm1, %xmm0, %xmm0
	vinsertf128	$1, %xmm2, %ymm0, %ymm0

On haswell it's a simple
	vpmovzxbw	%xmm0, %ymm0

There is a maze of duplicated and dead transforms and patterns in this
area. Remove the dead custom lowering of zext v8i16 to v8i32, that's
already handled by LowerAVXExtend.