[llvm] [llvm-exegesis] Begin replacing unsigned with MCRegister. NFC (PR #123109)

[Min-Yih Hsu via llvm-commits]

================
@@ -46,7 +46,7 @@ struct RegisterAliasingTracker {
 
   // Construct a tracker from an MCPhysReg.
----------------
mshockwave wrote:

```suggestion
  // Construct a tracker from an MCRegister.
```

https://github.com/llvm/llvm-project/pull/123109