This patch proposes we drop the TuningPrefer256Bit tuning flag on Ice Lake Server targets and later.
It doesn't appear that IceLake and later CPUs have the drastic frequency scaling issues with 512-bit vector usage, unlike earlier AVX512-capable targets: https://travisdowns.github.io/blog/2020/08/19/icl-avx512-freq.html
After internal testing by @pengfei I limited this to just the Server variant - Ice Lake Client, Rocket Lake and Tiger Lake targets still prefer 256-bit vectors at this time.