[llvm] [LLVM] Insert IMPLICIT_DEF for a register sequence if any operand is undef (PR #158000)

[Matt Arsenault via llvm-commits]

================
@@ -1998,9 +1999,17 @@ void TwoAddressInstructionImpl::eliminateRegSequence(
                   .valueOut();
     }
   }
-
+  for (unsigned i = 1, e = MI.getNumOperands(); i < e; i += 2)
+    if (MI.getOperand(i).isReg() && MI.getOperand(i).isUndef()) {
+      // Insert the IMPLICIT_DEF on dst register.
+      MachineInstr *DefMI =
+          BuildMI(*MI.getParent(), MI, MI.getDebugLoc(),
+                  TII->get(TargetOpcode::IMPLICIT_DEF), DstReg);
----------------
arsenm wrote:

This is emitting a full register def for each input. You only need to insert a subregister def for this particular subregister from the reg_sequence input

i.e., something like
```
BuildMI(IMPLICIT_DEF)
  .addDef(DstReg, 0, MI.getOperand(I + 1).getSubReg())

```


https://github.com/llvm/llvm-project/pull/158000