[llvm] [Mips] Print ABI register names in MIR (PR #228017)

[Jiaxun Yang via llvm-commits]

FlyGoat wrote:

> Please just adopt the numbered register names. MIR should just follow the TableGen names. We don't need this hook just to make MIR marginally friendlier to read.

My personal data point is that, after working on MIPS low level software and compilers for more than seven years, I still can't get my brain around numbered register names. `$ra` being `$31` and `$sp` being `$29` are about all I can remember.

Since MIR is primarily useful for debugging CodeGen which calling convention matters,  I think recognizable register names have real debugging value rather than being merely cosmetic.

That said, if the community feels the additional MIR hook isn't justified, I'm fine with adopting numbered names.

Cc: @willir if MediaTek folks have further comment.

https://github.com/llvm/llvm-project/pull/228017