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

[Ryan Cowan via llvm-commits]

HolyMolyCowMan wrote:

Split into 3 PRs: this one for FPTRUNC, https://github.com/llvm/llvm-project/pull/165554 for FPEXT and https://github.com/llvm/llvm-project/pull/165539 for removing unnecessary `mov` instructions.

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