[llvm] [CodeGen] Use non-static Register::virtRegIndex() instead of static Register::virtReg2Index. NFC (PR #125031)

[Matt Arsenault via llvm-commits]

================
@@ -187,10 +187,10 @@ struct PPCMIPeephole : public MachineFunctionPass {
 
 #define addRegToUpdate(R) addRegToUpdateWithLine(R, __LINE__)
 void PPCMIPeephole::addRegToUpdateWithLine(Register Reg, int Line) {
-  if (!Register::isVirtualRegister(Reg))
+  if (!Reg.isVirtual())
     return;
   if (RegsToUpdate.insert(Reg).second)
-    LLVM_DEBUG(dbgs() << "Adding register: " << Register::virtReg2Index(Reg)
+    LLVM_DEBUG(dbgs() << "Adding register: " << Reg.virtRegIndex()
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arsenm wrote:

```suggestion
    LLVM_DEBUG(dbgs() << "Adding register: " << printReg(Reg)
```

https://github.com/llvm/llvm-project/pull/125031