[llvm] [CodeGen] Refactor DeadMIElim isDead and GISel isTriviallyDead (PR #105956)

[Matt Arsenault via llvm-commits]

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@@ -119,8 +106,18 @@ bool DeadMachineInstructionElimImpl::isDead(const MachineInstr *MI) const {
     }
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arsenm wrote:

Why doesn't the isPhysical path consider the dead flag on the operand? 

https://github.com/llvm/llvm-project/pull/105956