[llvm] AMDGPU/GlobalISel: RegBankLegalize AGPR support and gfx908 MFMA rules (PR #192603)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> rule looks fine, but I am more interested what to do with agpr copies. @arsenm Here we apply vgpr for divergent values, do we want to implement some "spread agpr to uses" in some way to get similar behavior as generic regbankselect

Yes, though you have very little freedom on 908, and for 90a+ the AGPR usage is a problem for much later in the flow. The only thing to do here is ensure phis use AGPRs depending on MFMA uses 

https://github.com/llvm/llvm-project/pull/192603