[llvm] [LangRef] Normalize IEEE 754 spelling (PR #174721)

[Ben Dunbobbin via llvm-commits]

bd1976bris wrote:

> This makes this doc consistent. I notice there is a mixture of spellings throughout the code though. Perhaps using the `IEEE754` spelling here would be best as that can be used consistently everywhere?

I didn't mean to block this review. This was meant to be a helpful take-it-or-leave type suggestion. I'm happy with the patch as it is.

https://github.com/llvm/llvm-project/pull/174721