[llvm] [Thumb1] Resolve FIXME: use 'mov hi, $src; mov $dst, hi' (PR #81908)

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AtariDreams wrote:

> r10 and r11 are callee-save, so you'd need to make sure they get spilled

r10 and r11 are callee-save, so you'd need to make sure they get spilled

I only think this is an issue if the compiler were to randomly switch between thumb and arm mode and vice versa mid function, which is a thing that only really happens in handwritten code anyway. 

https://github.com/llvm/llvm-project/pull/81908