[llvm] [AArch64][GlobalISel] Use TargetConstant for shift immediates (PR #161527)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Thanks this was helpful. I'm not sure if I see the reason for the split between constant and targetconstants - it feels a bit nit picky for something that doesn't buy us a lot. It is something that we can make work though, I will update the patch.

It's very important. Constant is materialized in a register, and TargetConstant is not 

https://github.com/llvm/llvm-project/pull/161527