[X86][AVX512] Use WriteVPMOV256 sched class for all truncations/extensions.

At the moment these are identical to WriteShuffle256 (which we were using), but it should be WriteVPMOV256 to match the AVX2 instruction, plus it will help us remove some unnecessary overrides by tweaking the WriteVPMOV256 class.

Also, as D115547 shows, we still need to split off 128-bit extensions/truncations sched classes to remove some other overrides.