[llvm] [ARM][KFCI] Fix unused variable for #163698 (PR #164857)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Actually, looking at this case, the guidelines in the coding standards are to inline the call. So we just end up with:
```c++
    assert(ARM_AM::getT2SOImmVal(imm) != -1 &&
           "Cannot encode immediate as Thumb2 modified immediate");
```

This was changed recently (discourse post should be easy enough to find). But `NDEBUG` ifdefs I don't think were ever preferred in most areas of the repo.

https://github.com/llvm/llvm-project/pull/164857