[llvm] [AMDGPU] Legalize SGPR hi16 copies into S_PACK_HH_B32_B16 (PR #218371)

[Arseniy Obolenskiy via llvm-commits]

aobolensk wrote:

> Fixing this in SIFixSGPRCopies feels wrong. A copy from SGPR hi16 should probably never have been created in the first place. How did it get created?

Probably, but as of now SGPR hi16 is being generated in isel after selecting https://github.com/llvm/llvm-project/blob/20693469df1207f92076a9f2b49cb0fb28a7f20e/llvm/lib/Target/AMDGPU/SIInstructions.td#L2523-L2527

https://github.com/llvm/llvm-project/pull/218371