[llvm] [CodeGen] NFC: Move isDead to MachineInstr (PR #123531)

[Matt Arsenault via llvm-commits]

================
@@ -1786,6 +1787,13 @@ class MachineInstr
   /// Return true if all the implicit defs of this instruction are dead.
   bool allImplicitDefsAreDead() const;
 
+  /// Check whether an MI is dead. If \p LivePhysRegs is provided, it is assumed
+  /// to be at the position of MI and will be used to check the Liveness of
+  /// physical register defs. If \p LivePhysRegs is not provided, this will
+  /// pessimistically assume any PhysReg def is live.
+  bool isDead(const MachineRegisterInfo *MRI,
+              LiveRegUnits *LivePhysRegs = nullptr) const;
----------------
arsenm wrote:

Should move this next to wouldBeTriviallyDead, and preferably explain the difference between them 

https://github.com/llvm/llvm-project/pull/123531