[llvm] [GlobalISel] Allow Legalizer to lower volatile memcpy family. (PR #145997)

[Pete Chou via llvm-commits]

petechou wrote:

> LGTM. This wasn't your doing but it's strange why the original commit that added the SDAG check lines to the mops test didn't use the same optimization level as GISel (O2 vs O3): [5aa08bf](https://github.com/llvm/llvm-project/commit/5aa08bf70854fc1fb2d595ce12813fe5f40494dc)

@aemerson Thanks for approving the PR! Could you please also help merge it as I don't have the write permission. Thanks.

https://github.com/llvm/llvm-project/pull/145997