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

[Matt Arsenault via llvm-commits]

================
@@ -1706,6 +1600,19 @@ bool RegBankLegalizeHelper::applyMappingDst(
       assert(RB == getRegBankFromID(MethodIDs[OpIdx]));
       break;
     }
+    case VgprAnyTy: {
+      assert(RB == VgprRB);
+      break;
+    }
+    case AgprAnyTy: {
+      if (RB == AgprRB)
+        break;
+      Register NewAgprDst = MRI.createVirtualRegister({AgprRB, Ty});
+      Op.setReg(NewAgprDst);
+      if (!MRI.use_empty(Reg))
----------------
arsenm wrote:

```suggestion
      if (!MRI.use_nodbg_empty(Reg))
```

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