[llvm] [Mips] Support "$sp" named register (PR #136821)

[via llvm-commits]

yingopq wrote:

> Looks good for adding support for `$sp`. Should we other general purpose registers as well? The Clang frontend accepts `$1` through `$31` along with their aliases. Currently,`$28`, `sp` and `$sp` work here, but the rest still cause a crash.

We can see the code comments `The Linux kernel uses $28 and sp`, so we keep consistent with the Linux kernel. May I understand it correctly?

https://github.com/llvm/llvm-project/pull/136821