[llvm] [CodeGen] Use MCRegister for CCState::AllocateReg and CCValAssign::getReg. NFC (PR #106032)

[Sergei Barannikov via llvm-commits]

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@@ -116,7 +116,7 @@ static bool f64RetAssign(unsigned ValNo, MVT ValVT, MVT LocVT,
   static const MCPhysReg HiRegList[] = { ARM::R0, ARM::R2 };
   static const MCPhysReg LoRegList[] = { ARM::R1, ARM::R3 };
 
-  unsigned Reg = State.AllocateReg(HiRegList, LoRegList);
+  MCRegister Reg = State.AllocateReg(HiRegList, LoRegList);
   if (Reg == 0)
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s-barannikov wrote:

```suggestion
  if (!Reg)
```

https://github.com/llvm/llvm-project/pull/106032