[llvm] [AMDGPU] Add support for HW_REG_WAVE_SCHED_MODE (PR #169840)

[Jay Foad via llvm-commits]

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jayfoad wrote:

Nit: suggest moving it up to line 526 since this section is really only for IDs that meant something different before GFX12.

https://github.com/llvm/llvm-project/pull/169840