Restructure x86 signed pack instructions

[Nilstrieb]

This reduces the amount of duplicated code and the chance for bugs.

Sorta replaces #1442. I'm not very happy with how it turned out, but it looks mostly decent I think.

Also adds new support for _mm_packs_epi16, _mm256_packs_epi16 and _mm256_packus_epi32.

For reference, here's a table of all the instructions:

	sse 16	avx 16	sse 32	avx 32
unsigned	_mm_packus_epi16|llvm.x86.sse2.packuswb.128	_mm256_packus_epi16|llvm.x86.avx2.packuswb	_mm_packus_epi32|llvm.x86.sse41.packusdw	_mm256_packus_epi32|llvm.x86.avx2.packusdw
signed	_mm_packs_epi16|llvm.x86.sse2.packsswb.128	_mm256_packs_epi16|llvm.x86.avx2.packsswb	_mm_packs_epi32|llvm.x86.sse2.packssdw.128	_mm256_packs_epi32|llvm.x86.avx2.packssdw

[bjorn3]

Thanks!