[llvm] Remove unneeded work from machine memset (PR #65672)

[Fangrui Song via llvm-commits]

MaskRay wrote:

This needs an AArch64 tag and a NFC tag.

`[AArch64] Remove unneeded work from mops memset. NFC`  may be a better title.

When committed, this sentence "I noticed some unnecessary computation in aarch64." can be removed from the description.

https://github.com/llvm/llvm-project/pull/65672