[llvm] [X86] Add tuning features for Hygon C86-4G-M7 (PR #205992)

[Xiaomeng Zhang via llvm-commits]

zhangxiaomeng-hygon wrote:

> A lot of these extra slp RUNs seem excessive?

I noticed that SKX has TuningPrefer256Bit enabled and already includes SLP regression tests for it, so I've added similar regression tests for m7/m8 as well.

https://github.com/llvm/llvm-project/pull/205992