[llvm] [AArch64][GlobalISel] Add pattern to prevent scalar uqxtn fallback (PR #201546)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Seems OK to me. There is presumably a sqxtn and sqxtun too that could work the same way. Did you want to add them here or do it in a followup?

https://github.com/llvm/llvm-project/pull/201546