[llvm] [AArch64][CodeGen] Fix crash when fptrunc returns fp16 with +nofp attr (PR #81724)

[Harald van Dijk via llvm-commits]

hvdijk wrote:

This effectively includes my PR #80576, which is ready to merge except for an unresolved question about where and how to document the change. If I can get some guidance on what documentation is wanted there, I can amend that, push it, so that when we then rebase this one, it shows fewer changes?

https://github.com/llvm/llvm-project/pull/81724