[llvm] [RegAlloc] Sort CopyHint by IsCSR (PR #131046)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> > @arsenm I'm having some trouble coming up with a test case that isn't an exact clone of one of the existing tests, even after running through llvm-reduce.
> 
> Which case is this? You were trying llvm-reduce on the MIR? I can try to see if my out of tree patches help reduce it any

The AArch64 cases were good candidates because they reduced spillage, however they also required us to use a specific mattr (pauth) to get it. I was hoping for something more general. When we reduce them though, they are the same test.

The AVR case `foo2` is pretty simple and has less spillage, except the test case is pretty simple already. Is it worth to add a copy of this test in its own file to test this?

The X86 test cases do not reduce spillage, so I didn't think they were good candidates.

https://github.com/llvm/llvm-project/pull/131046