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

[YunQiang Su via llvm-commits]

wzssyqa wrote:

> > I think that we need something to support something like $0-$31 and something like $gp, $t1 etc.
> 
> You mean we need to process all registers?

yes.

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