[X86] Enable shuffle combining for AVX512 unless the root is used by a vselect

A lot of vectorized code doesn't use masks so we shouldn't penalize them by not doing shuffle combining on avx512 targets.

I've added support for VALIGNQ/VALIGND and 512-bit SHUF128 to prevent some regressions. I also prevented recombining 256-bit SHUF128 to PERM2X128. We may not need to add 256-bit SHUF128 support, but I don't think I found any cases requiring that in my testing.

Differential Revision: https://reviews.llvm.org/D77928