[clang] [llvm] [MIPS][ISel] Fix musttail (PR #161860)

[Folkert de Vries via cfe-commits]

folkertdev wrote:

There have been two other tail call PRs recently, that I think have some valuable additional test coverage:

- https://github.com/llvm/llvm-project/pull/168506
- https://github.com/llvm/llvm-project/pull/168956

In particular they test swapping arguments around, and using `byval` and `sret`.

https://github.com/llvm/llvm-project/pull/161860