[llvm] [IR] Add `llvm.modf` intrinsic (PR #121948)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm commented:

Sorry, I've been reviewing this PR on and off throughout the day and had some latent suggestions.  Otherwise I think this PR is good to go.

https://github.com/llvm/llvm-project/pull/121948