[llvm] [AArch64][GlobalISel] Improve lowering of vector fp16 fptrunc (PR #163398)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Thanks - Can we add something to explain the new opcode in llvm/docs/GlobalISel/GenericOpcode.rst

https://github.com/llvm/llvm-project/pull/163398