[llvm] [AArch64][GlobalISel] Select narrow GPR constants (PR #204322)

[David Green via llvm-commits]

https://github.com/davemgreen approved this pull request.

I was looking at getting shifts to extend the shift type later in the pipeline recently and it required code like this. It didn't work very well for other reasons, but we might want to make improvements to this area to help selection without RBS.

In the meantime this LGTM. Thanks.

https://github.com/llvm/llvm-project/pull/204322