[llvm] Use 256 bit register when AVX2 or higher is available. (PR #91721)

[via llvm-commits]

shamithoke wrote:

> I can't see anything in Agner/instlatx64/uops.info to match this, and it'd be very unusual for a 256-bit instruction to be faster than the 128-bit variant.
> 
> Do you have a benchmark that I can run on my rocketlake machine?
> 
> @phoebewang Do you have any ideas what could be going on?

I have a java test (that is taken through an internal JIT compiler) and I measured performance for this test. 
Can you please recommend what kind of benchmark can be built?

I observe roughly 5% improvement while using AVX2.

https://github.com/llvm/llvm-project/pull/91721