[llvm] [AMDGPU] RewriteMFMAFormStage: use isMFMA instead of isMAI (PR #199976)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I got it wrong earlier: the SIFoldOperands pass can emit concrete V_ACCVGPR_READ_B32_e64 instructions. But in that scenario, even though the dst of V_ACCVGPR_READ_B32_e64 is a VGPR, it won’t be used by anything related to this rewrite.

I believe it only does that for gfx908 where it's complicated to copy between VGPR and AGPR. For gfx90a+, these should be COPY 


https://github.com/llvm/llvm-project/pull/199976