[llvm] [AMDGPU] Have VCC as a first-class member of the SGPR pool. (PR #173870)

[Ivan Kosarev via llvm-commits]

kosarev wrote:

@arsenm The whole point of the change (and the linked ticket) is to recognise VCC as part of the 'numbered SGPRs', which it actually is, as opposed to it being just another special register that can be encoded as an SReg. I've just forwarded you the corresponding email discussion.

https://github.com/llvm/llvm-project/pull/173870