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

[Matt Arsenault via llvm-commits]

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arsenm wrote:

I'm not happy that we seem to have grown another place that needs these manual updates on opcode changes, but in this case you do not need a new opcode 

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