[llvm] Newpm/expand large fp convert (PR #71027)

[Matt Arsenault via llvm-commits]

arsenm wrote:

Manually pushed squash with fixed commit message in 3cef582ae430fb912f1d089fab7c64e77d40fcf9. Not sure what the best solution was, with the stack commit it has a messed up title and comment 

https://github.com/llvm/llvm-project/pull/71027