[llvm] [AMDGPU] Add test to show failure with SRC_*_HI registers. NFC. (PR #154828)

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

> I guess we need those no-subregister subclasses?

We still need it for liveness, but _hi must be marked as isArtificial. This is not easy though and likely need some fixes in tablegen backend. But I can confirm that stops the coalescer.

https://github.com/llvm/llvm-project/pull/154828