[llvm] [LoopUnroll][NFCI] Clean up remainder followup metadata handling (PR #165272)

[Joel E. Denny via llvm-commits]

jdenny-ornl wrote:

> LGTM. One of the `makeFollowupLoopID` indeed seems redundant.

Thanks for reviewing!

> Since we are not completely sure this is dead code (do did you find it?),

I stumbled over it when adding [this setLoopEstimatedTripCount call](https://github.com/llvm/llvm-project/pull/159163/files#diff-a1425e9579dc23903005a96304da3db353b3475db5ae4772f0cc4667cb445c41R521).

> maybe use NFCI in the title.

Done.

https://github.com/llvm/llvm-project/pull/165272