[llvm] [Mips] Add set directive support for EVA instructions (PR #225888)

[Anand Mistry via llvm-commits]

akmistry-msft wrote:

> Please amend pr text body for some description as this will become commit message once committed. code LGTM.

Done. Added a short comment, since the change is fairly simple.

https://github.com/llvm/llvm-project/pull/225888