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

[Nashe Mncube via llvm-commits]

nasherm wrote:

> Thanks. This looks OK to me with a little bit of cleanup so long as others dont disagree. Make sure you update the strictfp_f16_abi_promote.ll test.

Rebasing onto @hvdijk merged PR updates updates strictfp_f16_abi_promote.ll and reduces the size of this PR. Hopefully addressed all outstanding comments

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