[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:

LiveRangeCalc requiring this pass run before is spaghetti. We shouldn't need this pass to fix the liveness representation. The liveness of physical registers should be fully expressed at all points in the compilation. This pass has been papering over issues in LiveRangeCalc 

The implicit-def of w2_hi is redundant with the x2 implicit-def. The use was marking a kill (except it didn't add a kill flag?). So there's no reason to have the new implicit-def, and losing the implicit use loses the use kill point 

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