[llvm] [AMDGPU][MC] Disallow null as saddr in flat instructions (PR #101730)

[Matt Arsenault via llvm-commits]

================
@@ -6244,7 +6248,11 @@ void SIInstrInfo::legalizeOperandsFLAT(MachineRegisterInfo &MRI,
   if (moveFlatAddrToVGPR(MI))
     return;
 
-  Register ToSGPR = readlaneVGPRToSGPR(SAddr->getReg(), MI, MRI);
+  const TargetRegisterClass *DeclaredRC =
+      getRegClass(MI.getDesc(), SAddr->getOperandNo(),
+                  MRI.getTargetRegisterInfo(), *MI.getParent()->getParent());
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arsenm wrote:

```suggestion
                  &RI, *MI.getParent()->getParent());
```

https://github.com/llvm/llvm-project/pull/101730