[llvm] [AMDGPU] Adapt new lowering sequence for `fdiv16` (PR #109295)

[Shilei Tian via llvm-commits]

shiltian wrote:

> Can you explain the algorithm? Or at least add a comment showing _what_ the algorithm does in pseudocode, so I don't have to work it out from all the getNode calls?

Done. Please refer to the ticket (as well as the Gerrit PR attached to the ticket) for more details.

https://github.com/llvm/llvm-project/pull/109295