[llvm] [MC][Mips] Fix wrong assumption about `Immediate` operand. (PR #119056)

[Romain Thomas via llvm-commits]

romainthomas wrote:

>From what I understand, most of the logic of Mips disassembler testing is done in `llvm/test/MC/Disassembler/Mips/`. There is also some disassembler unittests in `llvm/unittests/MC` but **not** for the Mips Target (only `AMDGPU, X86, SystemZ`).

I don't think we could exercise this code path with a lit test[^1] using `llvm-mc` so a unittest would be more appropriated. But since there is no unittest for Mips it would require bootstrapping them. 

Who should own this bootstrapping? Me in this PR? Me in a different PR? The Mips code owners? 

[^1]: I can be completely wrong on this point

https://github.com/llvm/llvm-project/pull/119056