[llvm] [LLVM][Docs] Define IR in the lexicon (PR #221388)

[Prajit Rahul via llvm-commits]

Prajit-Rahul wrote:

> LGTM. Thanks and welcome! Let me know if you would like me to merge your PR on your behalf.

Thanks for the review! Yes, please merge it on my behalf.

https://github.com/llvm/llvm-project/pull/221388