[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.
   RegisterAliasingTracker(const MCRegisterInfo &RegInfo,
-                          const MCPhysReg Register);
+                          const MCRegister Register);
----------------
mshockwave wrote:

I have been pondering this part of the change: on one hand many of the callers (of RegisterAliasingTrackerCache::getRegister) have been calling with unsigned so change this to MCRegister matches that expectation; but on the other hand, I don't think we track virtual register here in RegisterAliasingTracker, which is why it was MCPhyReg.

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