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

[Reid Kleckner via llvm-commits]

rnk wrote:

I like this as-written. I don't think inlining the call into the assert harms the readability, and it's arguably more efficient, i.e. we don't do the encoding function call at all. It's not clear to me if the optimizer can remove that.

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