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

[Simon Pilgrim via llvm-commits]

RKSimon 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?

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