[llvm] [LiveVariables] Mark use as implicit-def if defined at instr (PR #119446)

[Matt Arsenault via llvm-commits]

================
@@ -277,11 +277,13 @@ void LiveVariables::HandlePhysRegUse(Register Reg, MachineInstr &MI) {
           continue;
         if (PartDefRegs.count(SubReg))
           continue;
+
+        // Check if SubReg is defined at LastPartialDef.
+        bool IsDefinedHere = LastPartialDef->modifiesRegister(SubReg, TRI);
         // This part of Reg was defined before the last partial def. It's killed
         // here.
-        LastPartialDef->addOperand(MachineOperand::CreateReg(SubReg,
-                                                             false/*IsDef*/,
-                                                             true/*IsImp*/));
+        LastPartialDef->addOperand(
+            MachineOperand::CreateReg(SubReg, IsDefinedHere, true /*IsImp*/));
----------------
arsenm wrote:

> we just need to remove the implicit use.

So can you just skip the add operand altogether? I'm still confused about why this pass is doing anything. This is very old code that predates subregister tracking. I bet we can delete all of this code as a step towards deleting the entire pass

https://github.com/llvm/llvm-project/pull/119446